Abyss Ambulator
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The Rules and Exceptions Paradigm
The Paradigm
Many, if not most, games follow the Paradigm of Rules and Exceptions. The game is constructed around a general set of Rules, that apply to most situations, and a set of special Exceptions, where the Rules can be broken.
It is a matter of definition whether an entertainment activity without at least vaguely formulated Rules, such as story-telling perhaps, can be a game. Either way, such activities are ignored here.
Examples
Examples of the Paradigm abound. Take chess. An important Rule is that players take turns moving one piece at a time. The obvious Exception is castling, where a player is allowed to move both their king and rook on the same turn, provided certain requirements are met.
Or take Super Mario Bros.. The general Rule is that touching enemies hurts Mario. There are several Exception. One is invincibility, granted by a Starman. Once invincible, the Rules are inverted and Mario kills enemies by touching them. Another Exception is that some enemies can be defeated by jumping onto them, but not walking into them.
Or take Scythe, a really excellent modern boardgame. A core aspect of each faction is the ability to break a specific general Rule. Repeat actions are generally forbidden, except for the Rusviet faction. Workers can't cross rivers unaided, except Nordic ones can. According to the designer, this approach is deliberate and intended to emphasise the most important Rules of the game. This works well.
Negative Exceptions
Exceptions can also be negative. Take chess again. Normally, a player has the choice to move any of his pieces, so long as they have somewhere to go. But in some cases a player is restricted to a subset of moves, for example in the case of check. In extreme cases towards the end of the game, sometimes only a single legal move remains, meaning the player has no choice at all.
Negative Exceptions can be used to increase the difficulty of games, in some cases acting as a handicap. A good example of this is the ancient Chinese boardgame Go, variously known as wéiqí, igo or baduk, recently brought to wider public attention when the narrow AI AlphaGo beat top-ranking professional Lee Sedol. The most basic Rule is that players alternate in placing single stones on the board. The most common handicap system in use grants the weaker player the Exception to place multiple stones in succession at the start of the game.
Simple and Complex Games
Some simple games are all Rules and no Exceptions. Tic Tac Toe and Rock, Paper, Scissors are obvious examples. There are so few possibilities that special cases simply do not exist.
Other games are intricate webs of Exceptions, characterised to a greater or lesser extent by the interaction between special cases, rather than between an Exception and its related Rule.
Obvious examples include long-running collectible card games, such as Magic: The Gathering or Android: Netrunner. In both, the designers regularly need to publish, and sometimes revise, specific legalistic rulings that explain and justify special interactions. In order to manage the load of interactions, especially when it comes to organised play, only a subset of cards is usually tournament-legal at any one time. This also happens to be a useful commercial strategy.
Netrunner attempts to address this concern by introducing the so-called 'Golden Rule': "If the text of a card directly conflicts with the rules in this book, the card text takes precedence." Of course, this does not resolve conflicts between cards.
This complexity often leads to heated debates between the involved parties, such as players, judges and game designers. More worringly, for the designers, this can lead to games being played 'wrongly', with negative effects on entertainment, fairness and replayability, to name just a few.
Other examples are pen and paper RPG's, such as Shadowrun, that are often stitched together from a variety of source-books, with a plethora of Exceptions and sometimes even contradicting Rules.
Roguelikes and Crawl
Roguelikes tend to fall on the side of intricate interactions. Dungeon Crawl Stone Soup, Crawl for short, follows this trend. There are a large number of items, item categories, item properties, mutations, spells, gods, enemies and so on. There are many subtle differences.
Take consumables for example. What is the difference between potions and scrolls? The effects are different of course, but the differences between potions seem about as large as the difference between the two groups. The same is true for scrolls. The main distinctions are subtle and rare, and sometimes not universal. Mummies can't quaff potions, scrolls can't be read when silenced, the blurry eyes mutation only interacts with scrolls, access to scrolls and potions can be sacrificed separately by worshippers of Ru. The differences appear to be entirely Exceptions, while the general Rules don't create any distinction except in name.
Species and Gods
Perhaps the two most consequential decisions a player makes in each game is the initial choice of species and the later (or sometimes concurrent) choice of god. This is closely followed by background, but its effect fades continually throughout the game. While the background choice is of great significance in the early dungeon, it drops to moderate significance by lair and is largely irrelevant in the late and extended game.
Species and god choice on the other hand remain significant throughout the entire game, although this effect is not constant either. Some gods wane in power, like Yred and perhaps Fedhas, and others are significantly stronger in certain areas of the game, like TSO in Pandemonium and Fedhas in Swamp. Similarly, some species start strong and stay strong, such as Centaurs and Deep Dwarves, while others start weak and remain so for some time, like Octopodes who suffer from a lack of armour. Some species can eventually manage to overcome some of their draw-backs, such as Formicids and Nagas joining a religion that gives access to a strong escape ability.
Species (and gods) are largely formulated around Exceptions. Many of the most memorable and interesting aspects of races and religions are implemented via significant diversions from the general Rules. Species (and gods) are most interesting when they offer a unique experience. This is easiest to achieve by allowing a specific race or religion to break or even invert a specific general rule. The following section will briefly look at each species in turn to identify the significant Exceptions. Some less defining aspects will be mentioned by way of contrast. This may clarify the kind of things that make species and gods unique and interesting.
Aptitudes
Aptitudes vary between between all species and therefore require a special mention. Historically, several species were largely distinguished by their aptitudes. For example, compare Humans, High Elves and Mountain Dwarfs. The general Rule appears to be that each species must have a unique aptitude set. Although in more recent versions aptitudes have become flatter, with the obvious aim to allow more choice in playstyle.
Exceptionally high aptitudes in certain categories are defining aspects of species. What Merfolk would not choose polearms, what Ogre not Clubs? Average or low aptitudes do not contribute much to differentiate species because they are very wide-spread and rarely effect games significantly. Why would a player spend significant amounts of XP on a low aptitude skill for little return?
Centaurs
The first crawl species in the alphabet are Centaurs. The defining Exception granted to these horsemen is that they can move fast. The basic Rule is the 'Speed 10 Rule': 'It takes 10 aut to move one space.' Centaurs only need 8 aut. This changes gameplay significantly and makes centaurs one of the strongest species. For example, by being able to walk away from Adders in the early game, while most characters have to fight. Centaurs also have the best aptitude for bows in the game. This is significant as the faster movement speed can be used to kill dangerous enemies, such as Ogres and Ettins, at range without taking any damage. All other aspects could be changed without significantly affecting how the species plays. For example, giving access to Bardings instead of boots appears to be motivated by the flavour behind the species, rather than by pressing game design needs. Centaurs would still be strong without Bardings.
Deep Dwarves
Next up are Deep Dwarves. This sturdy species violates the Rule that, once out of combat, characters can wait until their health has regenerated to full. The Exception comes in the form of an innate mutation that sets natural health regeneration to zero. By way of compensation an ability to heal on demand is added, which is actually stronger than natural regeneration, because it can be used in combat. As if that wasn't enough, as a free bonus, Dwarves are also more resistant to many forms of direct damage. Since the healing ability can not be used an infinite number of times, this race is almost always forced into choosing at least one from a small set of god powers or necromancy to meet its healing requirements.
Deep Elves
Deep Elves are unexceptional. Their only outstanding qualities are their low health and their aptitudes: the highest proficiency in learning spell-casting and necromancy, and the joint-highest in charms, fire and ice. They also have the joint-highest MP. Combined with their relative fragility they were obviously designed to be archetypal magicians, but in practice there is a stronger choice for each background that starts with a book (Hill Orcs for fire, Deep Dwarves for necromancy and for earth, etc.). As far as making an interesting species centered on magic use, Deep Elves largely fail. There are many other potential Exceptions around spell casting that could be explored, such as MP regeneration, casting speed, cross-training, spell costs, power cap, hit chance, etc.
Demigods
Demigods are more interesting. The defining Exception of this species is its inability to worship any god. The general Rule is that any character can worship any god. Demigods are not the only Exception in this regard, but certainly the one that is most cleanly, comprehensively and obviously implemented. Considering the general strength of religions, Demigods are compensated with higher attributes, like strength and dexterity. In a similar way to aptitudes, this can not be considered a true Exception, rather Demigods are at the extreme high end of a non-linear scale.
Draconians
Draconians are one of a number of species that grow as they gain levels. On paper, this growth, or the development of a random breath attack, random innate resistance and permanent flight appear defining Exceptions, but in practice they do not create anything strikingly unique. Innate resistances exist in several other species and so does permanent flight. Further niche Exceptions are cold-bloodedness and a special interaction with the dragon form transmutation. The former can usually be mitigated easily, although there can be painful problems in the early dungeon like Gnolls with freezing polearms, and the latter is not used by many characters.
The Exception that has the biggest effect on how the species plays is the complete inability to wear body armour. This is a trait shared with Octopodes, but unlike blinged-out cephalods, Draconians have substantial innate armour class. The result is that Draconians make very good magic users, as they never suffer from encumbrance, have above average aptitudes, and good defences that cost substantially less experience to develop. It seems that the dragon flavour obscures, rather than enhances, this critical feature.
Demonspawn
The main Exception of Demonspawn is that they acquire a significant number of semi-random largely positive mutations. The impact of the different mutations varies substantially. Some are relatively inconsequential, such as talons or hooves, while others noticeably change gameplay, such as nightstalker, demonic guardian or powered by death. Due to the random nature of the mutations, some Demonspawn feel like weaker versions of other species, others feel like something quite different. The current incarnation of the potion of mutation allows most species to gain at least a moderate set of useful mutations, thus diluting the Demonspawn-experience.
Another notable Exception is that Demonspawn can't worship certain gods and are vulnerable to holy damage, traits they share with the undead species. Both Exceptions rarely impact gameplay significantly, until the player enters a holy-themed ziggurat floor or pandemonium level. Demonspawn also become increasingly vulnerable to silver, which has little practical relevance in most games due to the rarity of that damage type, but can sometimes be an unpleasant surprise. The idea behind the species was probably that players would adapt to their changing character, but in practice most of the mutations have too little impact. They largely lead to the occasional swapping of items, to shuffle around resistances or other properties.
Felids
Felids are a complex mixture of Exceptions. The most meaningful are the complete inability to wield weapons, wear armour or throw things, the increased movement speed, and the ability to gain extra lives in combination with low health. Bonus speed exists in other species and due to the way it effects the game, making kiting extremely powerful, distorts the way felids are played. The extra lives make this species relatively forgiving after the first few dungeon floors. It is functionally similar to a combination of Borgnjor's revification and teleporting.
The item use restrictions push Felids strongly into developing magic capabilities. Many species come with some item restrictions, but none go all the way to banning everything. There are also items that are available to everyone including scrolls, wands and books, leaving plenty of scope for future concepts. In many ways being unable to use most items results in a pleasingly stripped down version of the game, with a greater emphasis on positioning, movement and noise management.
Formicids
Formicids come with several unique Exceptions that alter gameplay substantially. The most noticeable is the permament stasis effect. Although it is not so much an Exception, as it is an extreme adherence to some general Rules, such as 'it takes 10 aut to move one space' and 'player characters can only move to empty adjacent spaces'. It is entirely conceivable that all races were under the effect of permament stasis in extremely early versions of the game, simply due to the absence of speed modifiers and translocation options. This leaves the character unable to use many of the best emergency tools, such as scrolls of blinking and teleportation, the swiftness and blink spells and potions of haste. Although it does protect from slow and paralysis, permanent stasis is still a significant handicap.
As partial compensation Formicids are given the ability to dig a shaft under themselves, instantly leaving the current floor and travelling to a random spot one to three floors deeper. Needless to say, this can be dangerous, particularly as it also causes noise. This ability can be used to good effect in Hell. Formicids are also the only species that is innately able to dig rock walls. This is important, as it allows for creating kill holes on most floors of the game, thereby making many enemies less dangerous.
Formicids are also the only species able to sense enemies from the start, due to their Antennae mutation, which makes it easier to avoid grouped enemies. The final Exception granted to these sentient insects is the ability to wield two-handed weapons together with shields, thereby eliminating one of the trade-offs between melee / ranged damage and defences. The overall effect of the listed Exceptions is one of the most interesting species. Strangely, Formicids don't feel particularly weak most of the time, even though they are significantly more difficult to win with than, say, Minotaurs.
The Undead
The three undead species, Ghouls, Mummies and Vampires, share several Exceptions, notably their inability to worship the three good gods or Fedhas, vulnerability to holy damage and dispel undead. Like their monster counterparts, they have access to torment immunity, which is otherwise only available via the necromutation spell, although Kiku can provide a high degree of protection. A rather unexpected Exception is that the undead races cannot cast several of the most powerful necromancy spells, such as Death's Door and Borgnjor's Revification, even though they have naturally good aptitude in that skill. It is not clear to me whether it is a design decision for balancing (i.e. being undead is considered to be powerful) or for flavour (i.e. the undead ended up in their current state by already having used powerful necromantic magic) reasons.
Another Exception shared by the undead is their non-standard interaction with food. Mummies need no food at all, which makes the game noticeably more convenient and smooth to play. No consideration needs to be given to the hunger cost of higher level spells, no inventory slots are sacrificed to rations, fruit or other delectables, there is no need to butcher corpses or carry around the resulting chunks. In the current version of the game, this freedom from starvation can be leveraged to spend very long periods of game-time on some levels, in order to continue gaining experience from spawning monsters, or, more obscurely, to gain more and more gifts from Xom.
The fact that this isn't usually, if ever, required outside of certain self-imposed challenges stops most, but certainly not all, players from this form of scumming. This can also be done with other species, but mummies certainly make it more convenient. Many discussions have been had about food as a clock, the removal of food without replacement and/or the introduction of a different clock to force the player to progress deeper into the dungeon. A study of a large number of mummy games will likely reveal that by far the majority are not hugely longer, in turns or real-time, than the games of most other species. If true, this would provide strong evidence for the pointlessness of food.
Mummies
The most significant Exception afflicting Mummies is their inability to quaff potions. Considering the power of many potions, such as Heal Wounds, Haste, Resistance and Curing, this is a noticeable handicap. It is worth noting that there is no species or god that strictly forbids reading scrolls, although Ru can demand a partial restriction. Although a number of status effects, such as confusion and silence, prevent the use of scrolls, but allow drinking potions. Due to the no-potions-Exception many Mummies are found today worshipping Gozag, a god that replicates potion effects. As an additional synergy Gozag offers no corpose chopping, which is entirely inconsequential for Mummies, while other species are sometimes forced to spend money on purchasing food. Finally, Mummies also have the second poorest aptitudes overall, except average fighting and necromancy abilities. In practice this can make Mummies feel weak until the point in the game when experience stops mattering.
Ghouls
Ghouls also don't have a traditional hunger clock. Instead Ghouls rot more the hungrier they are and can cure rot by eating meat. Eating also restores health, which was perhaps intended to counteract the fact Ghouls heal more slowly. Slower healing is not an Exception that affects the game noticeably, except in the extrem Dwarf case.
Ghouls are also one of three species that have an innate bonus to unarmed combat, in the form of claws, and are one of two species, the other being Trolls, that can relatively safely use it as their main form of attack; Felids are in most cases much better off with magic. Ghouls also start with the lowest total amount of attributes (with the exception of the currently experimental Gnoll species), including the lowest intelligence and joint-lowest dexterity. Similar to aptitudes, this is one end of a continuum, with Demigods being on the opposite end, rather than an Exception.
Vampires
The main Exception of Vampires is that their properties vary with satiation level. At normal and above satiation, this blood-thirsty species is more like a Human. At lower satiation levels it starts to become more like a cross between a Mummy and a Deep Dwarf. In practice, the lower satiation levels are more useful, as they grant free resistances, a stealth bonus and allow the transformation into a fast Bat, another notable and powerful Exception.
Due to the not completely controllable nature of satiation - Vampires sometimes gain satiation from attacking in melee - it can take time and effort to stay at the desired satiation level. It is relatively common to drop to the lowest level, bloodless, take some damage, and then need to drink blood to move up to thirsty, because vampires don't regenerate health when completely empty. Getting back to the bloodless level, if so desired, then takes several rounds of waiting. Needless to say, waiting isn't usually a fun part of games and this can interrupt the flow considerably.
Gargoyles
Gargoyles occupy the twilight zone between the undead and the living, like a Vampire somewhere between thirsty and bloodless. Like the living, Gargoyles interact normally with food. With the undead they share resistances to torment and poison. The most notable Exception of this stone species is a large innate bonus to armour, somewhat offset by lower health. While this could be used to make a relatively sturdy spell caster, many times players double down on the defensive advantage by wearing heavy armour, thereby achieving unparalleled protection from almost all attacks.
Gargoyles are also the only species inherently resistant to electrical damage. Another Exception is that Gargoyles cannot worship Yredelemnul, because they are neither living, nor (un)dead. This seems to be for purely cosmetic reasons, as there is no specific interaction between the god and the species that can be considered problematic. Interestingly, this mirrors the interaction between the real undead species and Fedhas. What is perhaps more strange is that the Statue Form spell allows any living species to become essentially a slower version of a Gargoyle. This is once again mirrored on the undead side with Necromutation allowing living species to effectively become Mummies. There is, as of yet, no spell to make the reverse possible.
Halflings
Halflings are, perhaps unsurprisingly, mainly a smaller version of humans, somewhat differentiated by aptitudes. They have, by some margin, the best aptitude for using slings. The only true Exception granted these little people is their innate resistance to mutation. Following the removal of the Amulet of Resist Mutation, this is now a fairly rare trait, if one considers the mutation immunity of the undead to be different. Considering that mutations are relatively rare in the game, and that there are a number of ways to avoid getting mutated, or to deal with bad mutations after accidental exposure, this resistance has little practical impact on the game.
Halflings are also one of the two small species. Once again, this is a point on a (semi-)continuum rather than a true Exception. The consequences are that Halflings can't use some of the larger two-hand weapons, treat some of the larger one-hander as two-handers and can't use large shields. It also grants a noticeable boost to evasion and stealth.
Kobolds
Kobolds are the other small species. Not only are Kobolds the only species that have a chemical element named after it, Cobalt, but it is also the species that is best with short blades and crossbows. In older versions of the game many species were largely distinguished by their aptitudes. Kobolds, Halflings, Deep Elves, Hill Orcs, Merfolk and Tengu are all in some ways holdovers from times past.
Merfolk
Merfolk represent water, one of the four elements of ancient Greek philosophy, as first described by Empedocles some 2,500 years ago. Gargoyles represent earth, Tengu air. In the current state of the game perhaps Hill Orcs come closest to representing fire, although other cases could be argued. In the past Lava Orcs were the obvious contender.
Merfolk are the faster of the two aquatic species. In a case of rule inversion, these fish-like beings speed up in water, while almost everything else slows down. As if that wasn't enough, Merfolk can even move through deep water and they also gain bonuses to defences and stealth while swimming. Merfolk are also the only species that can toggle an armour slot at will, outside of transmutation spells, by stepping into or out of water.
Except for their fishiness this species is only notably distinguished from humans by aptitudes. Merfolk have the highest aptitude to use Polearms and the joint-highest in Transmutations. In practice most Merfolk characters end up using polearms, while Transmutations are less commonly used in general. The advantages from being near water are powerful but apply very unevenly. Water isn't common in large parts of the game. However, there are several areas, notably the portal branch Sewers, the lair branches Swamp and Shoals, and the hell branch Cocytus, that positively overflow with water, making these substantially easier for sea-dwellers. In extreme cases Merfolk can kill enemies safely from a distance, taking no damage in the process. This can be entertaining when possible, but does feel like cheating. For maximum benefit the god Fedhas can be used to make water anywhere!
Minotaurs
If one species had to be chosen to represent recent versions of crawl then it would have to be Minotaurs. (In the past it might have been Mountain Dwarves. The difference is largely in the name in any case.) In particular the Minotaur Berserker combo is widely recommended to new players and has reached iconic status. It is perhaps surprising then that Minotaurs are not the strongest species. Instead the appeal probably lies in the simplicity.
In fact, Minotaurs might be the simplest species of all. This bull-headed species has the highest average aptitudes for melee and defensive skills. Combined with above average health and the second lowest (beaten by Trolls) average aptitudes for all things magic, this makes it extremely obvious which path to pursue: wear armour, pick any decent (non-short blade) melee weapon and attack enemies with it. The only restriction placed on Minotaurs is their inability to wear helmets, but that is more than made up for by their innate counter-attack. In games where backgrounds have long-term consequences Minotaurs would be the obvious fighters.
Notes
I started writing this sometime around 0.18 and stopped being engaged with the game around 0.20 or so. A good number of the claims may be outdated, but the spirit probably holds true.
I'm not currently playing, because I got sucked into other stuff.
The original intention was to do a grand review of all species and all gods and to draw some final conclusions, but alas that turned out to be very time-consuming. Some species are missing and no gods are explored.
As for the general conclusion: When designing stuff. think about the Rules and the Exceptions in explicit terms.
I randomly came across a draft and decided to just 'throw it out there' before it becomes even more irrelevant than it already is...
Apologies for the (probably) sub-par editing.
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