The lost Base

The lost Base is a Single Player level.

Walkthrough
Look to your right and enter the next room. Look through the hole in the broken Door to kill the 2 Enforcers, then turn around. Ignore the doorway to your left, instead heading to the upper left corner to collect the Nails beside some computers. Head into the alcove to your right, next to the computers, to collect a Nailgun. Look into the doorway you previously ignored, then jump into the next room. Head to the far side to drop into the Water below. Kill the Rotfish in front of you, then turn around and kill the 3 Rotfish behind you. Kill the Scrag in the upper left corner of the room, then turn around to kill the Scrag in the opposite corner. Turn left to kill an Enforcer on the balcony above you, then turn around to kill an Enforcer on the opposite balcony. Turn left, then swim to the doorway of the next room before climbing out of the Water.

Collect the Yellow Armor to your left, then turn right. Proceed forward to collect the Rocket Launcher, then continue forward to collect the Shells. Move slightly forward to collect the two 15 Health, then move forward once again to collect the 2 Nails and Rockets. Kill the Enforcer on the balcony above you, then turn around to kill an Enforcer on the opposite balcony. Turn right to look into the room, then kill the 2 Ogres on the platforms, 1 to your right and 1 to your left. Kill the 2 Enforcers on the balconies on the opposite side of the room, 1 to your left and 1 to your right. Proceed forward across the small bridge, be sure not to fall off into the Slime below as there is no escape. Collect the four 15 Health on the far side, two to your left and two to your right. Head to your right to collect the 2 Nails. Proceed forward, between a series of stalagmites, to collect a Super Nailgun. Head to the left wall on the opposite side of the room, then continue forward to pass a divider on your right. Collect the Double-Barrelled Shotgun, then continue forward to collect the 100 Health. Turn right, then move forward to reach an intersection before turning left. Kill the Vore in the darkness, then move forward before turning right.

Head to the wall before turning left. Kill the 2 Enforcers, 1 to your left and 1 to your right, before heading to the end of the corridor. Turn left, then head up the long ramp to reach the next room. Turn left to kill the Fiend heading towards you. Head to your right to collect the Shells, then continue forward to reach an intersection. Kill the 2 Enforcers and 2 Grunts heading towards you in the corridor to your right. Head down the corridor to your right before following a section straight ahead, then collect the Nails at the end. Retrace your steps to the intersection, then turn and head down the corridor you previously ignored. Kill the Death Knight at the far end of the room, then turn right to kill an Ogre in an alcove. Head into the alcove to collect the 100 Health, Cells, and Thunderbolt. Turn around and head out of the intersection, then turn right and head to the far side. Turn right to kill a Death Knight to your left and a Fiend to your right. Head to the middle of the room, then turn left to look through the doorway between the 2 Slime pillars. Kill the Shambler inside. Turn right and head to your right to collect the 15 Health beside the far wall. Return to the doorway with the Shambler and go through it, then continue forward to collect the Gold Key.