This section contains spoilers. Read it at your own risk.
Exit from your cell and proceed to your right. An ally will call you from the uppest level, but the stairs are blocked by some wooden boards. Get into the open cell right behind you, and pick up the bar near the corpse inside the tunnel. Break the boards and go upstairs. Enter the control room and pick up the smg and the armor, and your beloved friend will open the door and give you some ammo.
Proceed to the next block and fight your way through the Combine soldiers assault. Enter the second control room to go around the closed fence door. Restore yourself and proceed to the next hallway. You can choose if enter immediately into the next block or spend a little time to get down two Combine turrets. In this case, get your squad to a safe place.
Now proceed into the last block and help your fellowship against the Combine onslaught. Once you get past it, proceed and enter the hallway. There is a blocked door and a trip mine just behind that. To break it you have to toss a grenade through the broken glass. Grenades are in a crate near the end of the hallway, but look above your head for manhacks. The best solution to kill them is to quickly close the gate at the beginning of the hallway, let your squad destroy them and then reopen the gate and safely pick up all the grenades you need.
Enter the big room after the door and fight your way to the other side of it. Take advantage from the battle between Combine and antlions, leaving them kill each other. Once you are on the other side, your way is blocked by two trip mines. Make them explode so that your squad can continue following you. Now you have to fight against two ground turrets. You can go back into the big room and keep a metal barrel to protect yourself from turrets fire, sneak behind them and put them at rest with your favorite weapon.
To get through the combine wall, you have to enter the door at your left. Kill the soldiers guarding the console and press the button to disable the wall. If you need some extra ammo or health, go downstairs and proceed along the tight corridor until you get into a small patio (the one you saw just as you exit from your cell). Back to the combine wall area, be quick to keep control of the turret and prepare to reject the next soldiers onslaught.
Once you are finished enter the hallway. If you are short on allies, you'll get a friend to count on right inside the fence area. Proceed downstairs and get into the control room at your right. Enter the service corridor and kill the poison zombie waiting for you. Exit the service area and you are again in the room you just left, but behind the fence now.
Proceed into the next big watery room. The only enemies here are some barnacles. Be sure to kill them as your allies might actually be killed by them.
There are two optional puzzles here:
1. Get your squad pass over the broken passarel
2. Get into the room you see in this picture
For the first one, you can easily jump in the water and use the ladder to get to the other side of the room. There you'll find a valve to control the water level. Use it.
Secondary, you'll want to break the wooden boards blocking the big wooden cylinder under water, so that it can freely float and your squad can step on it.
As for the second, once you have used the valve, dive yourself beneath the broken passarel and look for a hole in the wall: get into it to emerge into the room. Restore yourself as you please and use the console to unlock the fence door.
Finally, bring your squad into the next room.
Open the door and get out. You are in a tight courtyard. There is an APC which will start to shot at you. Try to find a safe place for you and your squad, and resist to a massive Combine soldiers onslaught. At a certain point you will see some rockets fired from another APC. That one has been taken by prisoned citizens like you. These rockets should open a hole in the wall. Run into the building as fast as you can. There is a dead Antlion Guard in that service room. Look nearby: you'll find his bugbait-thing. Restore yourself a bit and enter the block.
Use your bugbait to get help from antlions. To navigate this block, please refer to the top view at you left. Once you have finished, proceed through the hallway. You'll find yourself into a large octagonal tower. The gate you want to enter is the one at your left, but it's closed, so proceed to the gate right in front of you, enter the control room and use the lever to open the gate.
Enter the gate you just opened. You'll have to fight against ground turrets and combine soldiers. Remember to use your friendly antlions to give your foes what they deserve. Eventually you'll get into the next big block. Again use the following picture to navigate it. Go down into next well, and enter a new service room where you'll find some ammo crate. Proceed again into the next room. Another puzzle here. See the black valve at your right? Use it to open the gas pipe. Now on the hanging electrical cable. Follow it outside and turn on electricity. Once you go back inside you'll see a big explosion that will open a wall into the wall.
Enter the infirmary. Have a quick look around and be ready to run away from an antlion guard. Take the long corridor and enter the control room at your left. Exit from the other side of the gate. The antlion guard will shove it. Now you have to face a few combine soldiers coming from the entrance at your right and from the ceiling. Let the antlion guard take really good care of them, and jump over the hole in the floor in front of you. Falling down means death.
Once you are in the courtyard (you remember it?) you can go downstairs on your right, and brake an item crate. Then proceed to the next level.
Enter the big courtyard, and prepare yourself for a great battle against two gunships. Keep you right and climb the blue trash dump to enter the area protected with fences and get the rpg launcher. Jump on the electrical boxes to exit and begin your fight. Kill the two gunships and all the soldiers. Once you are finished, a rebel will open the door that blocks your passage. Follow him into the next block. Now you have some red laser beams going up and down. You touch them and the ceiling turrets on the other side will certainly kill your friend. So keep your time and jump down. Proceed into the control room (don't forget to grab the AR2 near the body) where you can deactivate the beams and open the gate. Do it and wait for your friend. Go on and fight a small combine assault. If you want you can climb the ladder at your right and go into the small service room at the end of the catwalk. You'll find a couple of item crates but some headcrabs too.
Go upstairs and exit to the combine wall area. See the fire? There is an entrance through the wall nearby. Go there and jump down twice. Use the image here if you need extra ammo or health. Then proceed toward the combine pods. Just around the corner there are two ground turrets. Get rid of them, use the button on the combine console to open the wall at your back and get into the corridor. You are now entering the last level.
Go on and destroy the scanner coming for you. Go upstairs and enter the big room on your left. Taking cover behind the glass walls, use the console at your left to activate the pods rail above your head. Then get the elevator and proceed on the second floor. Get into the next room and get on one of the pod. Enjoy your ride until you fall down... You'll wake up in a service room. Go right and enter the hole in the metal wall behind the tank. Now you are finally out of Nova Prospekt, but it's not time to relax yet. Use the image at your left to navigate the bridge. Beware of the train and, of course, get down the straider on the coast. Once you are on the other side, just go downstairs and enjoy the view. Proceed to the dock (look for a simple easter egg nearby) and jump on the boat. Say goodbye to Nova Prospekt.
Thank you so much for playing.