Nightwatch (Link: www.hl-nightwatch.net)
, it should be sufficiently general for most mappers interested by this difficult and often overlooked aspect of mapping.
Loops are often a good idea. Let the player catch glimpses of where they are meant to be going. It can help to reduce any unpleasant linearity in your maps. Make getting straight through to those areas obviously impossible - it's no fun being taunted by the exit that's blocked off by a ledge that's just a tiny bit too high to jump.
Make the architecture appear functional. It shouldn't look like it was built solely for shooting monsters in - it's supposed to be a realistic portrayal of an actual place, whose architecture (ever so conveniently) aids or hinders the player's quest.
If a door has to be locked, make it obvious, and if it's to be unlocked, make that obvious too.
Tell a story. Why are things like this? What is the player doing? Make it look like something has happened, and give the player something to fight towards.
Make things hard, but fair. There's no fun in fighting a single headcrab with a rocket launcher; then again, the player is unlikely to enjoy taking on the entire US army with a crowbar.
Don't feel you have to fill the entire place with enemies. If there's a monster behind every corner, it destroys any kind of surprise. It's scarier when you have no idea what to expect (but don't put a gargantua in a cupboard, please...)
Use crates sparingly. Place items to be collected on shelves or even the floor, as it's not much fun having to smash your way through a cubic kilometre of crates to find a couple of bullets. If you do use crates, make sure you differentiate between breakable and unbreakable crates. They should look completely different - and be consistent!
Vents are again acceptable if used sparingly. They can be good for letting the player get into a room when the door's blocked, but not for mini-adventures lasting several hours apiece.
If you place a single snark in a vent, I will find out where you live, come round and forcibly re-educate you with an inflated kipper. Be careful with headcrabs in vents - they're obvious and so not-scary. Their absence would probably be terrifying...
Multi-sided battles can be great fun. Being caught in the middle of a firefight between soldiers and aliens really adds to the atmosphere, and makes gameplay much more unpredictable. Sometimes things go in your favour, other times you're in the midst of it. Sometimes you can sit back and relax until there's just a single headcrab left...
Compile early, compile often. Prevent potentially catastrophic scaling and gameplay errors by actually playing your map before it's finished.
Test with monsters in-place. Early on, scatter some appropriate monsters around your map. While they won't be the final enemies, they'll help you decide where things should eventually go, and can be invaluable in telling you which architecture works and which doesn't.
Use env_sound entities - when used properly, they can multiply spooky ambience by a factor of 17.41. Also, use ambient sounds where appropriate. They can contribute a lot to a map's atmosphere, but don't overdo it!
Let enemies react to what's happening. Don't just have independent rooms full of monsters - let soldiers call for backup, retreat or appear to respond to recent events.
Have things change in response to the player's actions. A good single-player map might have very few obvious scripted sequences, but there can still be a hell of a lot of scripting present.
Place AI nodes and place them well. A formerly stupid alien can behave more like Einstein when the AI node network is set up correctly. There are various impulse commands for displaying interconnections between nodes. Use them.
Tune your map. This can take weeks and can make or break a map. Adjust timings in all your multi_manages until things are spot on, tweak the positioning of monsters until they respond when the player is in just the right position and fix all minor bugs, glitches or oddities you find. A tenth of a second's timing or ten units in a single monster's postion can transform your map.
Here's a list of useful hints and tips relating to single-player mapping, in no particular order. Originally done for re-aligning multiplayer mappers working on