Wait a bit, then sprint back up and around the central section to the right - a part of the wall up to a lit area of the wall will lower, and you can sprint onto it to access a secret area with some Imps, marines and. Now go back to the stairs and sprint to the bottom. In a small tunnel into the central section of the room you'll find the yellow keycard. Clear the place out (there are some more Imps and shotgun marines, as well as a Demon and a Spectre at the back), and grab the goodies. At the top, snipe the shotgun zombies and Imps with your weapons, and head up. Anyway, go around back to the door and you'll find a wall section has opened, with a long staircase beyond it. You can open the wall section in the random alcove up here and shoot some enemies in the room beyond, but that's if you choose to go to the secret level or not (it's difficult, but fun). There's some ammo and armour up the lift, but otherwise go left at the bottom and you'll find a switch.
Go around and kill any Imps that come up the stairs, then go down and clear the place out. Head to the window on the right by the stairs, and you can shoot some more Imps and marines from up here. In this next room, quickly blast the central barrel to take out the two marines on either side, and then let the several Imps and marines/shotgun marines come into your field of view and blow them away with your shotgun.
