Up here, let your antlions deal with the guards, then approach the switch in the corner of the room overlooking the one you were in earlier, and press the switch to open the gate. Go up the stairs and at the top shoot the laser tripmine to get rid of it. Behind the two large boxes beneath the stairs is a supply crate containing a secondary AR2 charge (you now have the maximum 3 of these you can carry at any one time, so feel free to use one or two of them). Pull the filing cabinets away, and go out the door and through the newly-opened gate. Inside the office is some magnum ammo, and a switch you can throw to open the gate outside the office. Kill the guards here, then use the gravity gun to pull the plug out and get past. Snipe the guard who goes for the mounted gun. Get onto that floor and knock over the two turrets up here for your antlions, then head down the corridor. This is good, because there's nowhere else for you to go. You'll see the fence on the upper floor is bent, allowing you to crawl through the top of it. Follow the 'exit' sign (don't worry, you're not leaving this place for a LONG time) and go up the staircase as far as you can go. They still can't advance much further due to another turret up the stairs, but they'll either handle that themselves or you will in a moment. Out the door, you can see beyond the metal bars that a stationary Combine turret is killing your antlion friends as they come through the hole in the wall on the far side - do them a favour and knock the turret over with your crowbar or gravity gun. Jump into the hole, being wary of the headcrab that's just around the corner, and you'll come out in the control room. None of the open cells around here have anything in them, apart from the only lit one on the bottom floor which has a hole leading through to an old, battered control room. Head to the stairs at the far end and go down them.

Through the door you'll get a look at just what state the place is in - it's been around for a long time, that's for sure.

Start by breaking the two supply crates by the door to get some SMG ammo. Your first look inside the eerie Nova Prospekt.

Half life nova prospekt