Rule Zero. If you see a new monster,
walk away from it.'Walk away from it?'
On the tightest path towards the nearest known-safe upstairs, 10+ tiles, not moving near or through unexplored territory so you won't get sandwiched in from unexpected enemies. Also move to break LoS whenever convenient. Close doors behind you if only one enemy is following or it's a one enemy wide channel. (If you find yourself making movement mistakes (moving orthogonally along a corner corridor - two moves - instead of cutting the corner with a diagonal move - one move - and mistakes equivalent to that), calm down, it's a turn based game, not Run For Your Life Real Time Simulator. Assess, analyze, choose, repeat. A small amount of time spent to ensure your tactical movement is on-point will save hours of time dying to something you did not need to - a big win for both winrate and time spent playing Crawl.)
'Why should I obey Rule Zero? I walk towards every monster the moment I see it and hit it until it's dead! What's so bad about that plan?'
1) Let's talk about stealth, noise and monster AI. A monster starts
asleep. It remains asleep until either a noise (from fighting, a spell, a shout) reaches its ears, or it's in your LoS and you fail a stealth check (which occurs after every logical player turn). Then it has a possibility of shouting and it becomes
unaware. Unaware has two states:
Approaching noise (it walks to where the most recent noise it heard is) and
wandering (it hasn't heard a noise lately, so it moves at random). In this state, you can again fail a stealth check with it in LoS - it has a possibility of shouting and now it is
pursuing you and will actively move towards you until a) it's out of your LoS b) you pass a harder stealth check, whereupon it goes back to wandering. Also, you can make an aggressive action against the wandering monster - if you do this, it ALWAYS becomes pursuing, and NEVER shouts in the process. This trick has a name:
Shoutless. (Named by me because it didn't have one ^^)
...So, what does all this mean?
Notice there is no mechanic like: "If I am a group monster and I see a group member moving towards something, move with it." Nope! Monsters follow noises, are pursuing because they were in your LoS and saw you, or do nothing/wander around.
So let's say I see a
wandering orc. I grab its attention by throwing a stone at it, making it go to
pursuing without shouting. Maybe the action made a noise, and its other orc band members walk to where I threw the stone, so I must act immediately. I retreat away from the orc towards a nearby staircase and do so for 10+ tiles. Now even if other orcs heard the commotion - they go to it, see nothing, give up and go back to wandering. I can now dispatch this ONE orc at my leisure.
2) Similarly, if you rest or fight near the edge of LoS, unexplored territory could have monsters in it - including dangerous ones - and group monsters like orcs and gnolls could have friends near them - and the longer you fight/rest, the longer you give them to hear the noise of your recent actions, or simply just wander into your LoS because they were awoken by something. So also avoid doing this.
3) Stairs are incredibly OP. It takes 1.0 turns of 'leaving' before you leave the level, then you lose 1.333 turns (0.666 turns if you've never been to the level before) on the other side, but nothing that wasn't next to you when you started taking the stairs can follow. Stairs bring you to a level you know the layout of 100% and know is already cleared of monsters (respawns are possible, but they can happen anywhere and are very unlikely to be relevant), and more importantly - nothing can follow you now, you know that this is all you have to deal with (or escape from) now. If you retreat towards the stairs, you can use the stairs on less notice.
4) Every combat can go wrong. Every monster can get lucky hits, you can get unlucky miscasts, etc etc. If you are further away from unexplored territory, then less monsters can find you while you are weak from a bad encounter. You are closer to stairs that can save you from further conflict. Etc etc. Rule Zero is a simple way to minimize the risk you take while playing Crawl, by making less of it happen and ensuring you can flee from it better - because you've already done most of the legwork.
Follow Rule Zero until you've cleared Lair, and your win rate will shoot up! Doctors hate him! One weird tip discovered by a mom...(By the way, the next rule after rule zero to learn is:
You are not obliged to kill everything. Effectively, you 'kill' a monster by not having to fight it as much as you do killing it, because there is far more exp in the dungeon that is safe to acquire and you don't need to take risks to get more ever. Any time you flee from an orc priest and go to a different part of the level or different level, you have 'killed' it. And you can even come back and steamroll it at a higher experience level if it matters that much to you
)
(Appendix A: When a monster smites you, it deals 7-17 damage (weighted towards the average of 12). It is not resistable except for things like Deep Dwarf's
damage shaving - AC does nothing. When you see an orc priest, you should first think 'if he starts laying 17-bombs on my face on consecutive turns, do I have a plan for that? - maybe you don't, and you should run. Or if you HAVE to engage, just keep it in mind.)