How to create a scary atmosphere

[Wapez]

Do not forget to mix scary maps with a little bit more "safe" maps. Make the player feel rewarded for completing scary parts. It will also make the dark and tense maps feel even more unwelcoming.

Make the player feel like "I have to suffer through this to complete the puzzle", and not just "I don't know why I'm here". Giving places (and sometimes scares) a good reason will highly increase the level of which the player will feel like they're actually inside the game.

Dark convex corners, or other types of places where the player doesn't know anything until they crossed the line. Make the player think "I need to check every corner, so I don't get caught by surprise". Another trick for putting the player in the characters shoes.