Worked a bit on the first level. You encounter your first enemy after recieving a pistol from a fallen marine. You're in a maintenence room that uses red led lights which makes it extremely hard to see. There's a lot of noise too from the fuel pumps so it can cover up the sound of enemy projectiles. When you go to open the door to an elevater, a Hell Knight comes out and attacks you. There's also a secret teleport trigger that makes him teleport behind you. Epic ambush is epic.