There’s a medical station on your left and pick ups ahead. Take a left and kill the imp, then the trites, another imp and a chaingun commando behind you.
Continue and grab the armor and medkit to your right in the next area. Kill the revenant that spawns in then loop back round to finish more imps and wraiths. Kill the archviles that spawn in first then mop up the others. A lot of enemies spawn in here so just keep looping round and shooting till they’re done – remember to use the soul cube when they mount up. You can also go back and use that medical station near the entrance.
When you are ready go through the next door.
Hit the control panel on the left to call the bridge. Kill three cacodemons as you cross the bridge and then use the control panel to select Storage.
Head through the door into Storage and go left to kill an imp and grab some pick ups. Head round the corridor and kill three commandos, go into the room on your left and then take the top left door and kill a revenant and grab the pick ups. Turn round to kill an archvile and a wraith then grab the BFG ammo back in the Main Storage room and retrace your steps to the bridge. This time select Server Bank and kill another cacodemon as the bridge moves round.
Kill two imps as you enter then a commando through the next door; grab the medkit on your left then another imp comes from behind you and another commando ahead.
Go through the next door and kill the trites on your right, head down the stairs and kil an imp on your left. Turn right for some ammo then continue down, killing two more trites and an imp in an alcove on your right at the bottom. You’ll find a medkit in his alcove then go through the next door.
An imp will spawn on your left and there are some pick ups there. Turn right to kill another imp and then head into the office and grab Steve Tooloose’s PDA from the desk. You can watch a video on the monitor here too.
Go back up the stairs, kill a couple of imps and return to the bridge. This time you want to select Central Processing.
Kill the cacodemons as it spins round.
Grab the ammo and video on your right as you enter. Then go through the door for the boss fight against the mutated Sarge. Use everything you’ve got and take cover behind the pillars but don’t get too close as sometimes the floor drops away when Sarge fires on a pillar. He’ll use a BFG on you as well so be careful and keep strafing.
When he’s dead take his BFG and head into the alcoves for some pick ups. Finally head to the alcove he came out of and crouch down in the left corner to enter the tunnel. Drop down onto the pipe and jump down onto the walkway.
Grab the pick ups littering the floor to your left and then go and call the lift, enter it to exit the level.