Another technique is to provide limited ammo, forcing the player to figure out how to move forward without killing everything, which means there are enemies pur

Another technique is to provide limited ammo, forcing the player to figure out how to move forward without killing everything, which means there are enemies pursuing from behind as well. Open map designs (such as Doom II) were great for that, because an enemy you woke up at the beginning of the level could end up almost anywhere on the map, providing pressure from behind and creating tension when you had to backtrack. It's less interesting when you can afford to sweep through a level exterminating everything you see.