There are two hallways leading out of the intro room. Bust the wooden boards and exit thru the left door.
hint: If you take the right hallway you will find a battery behind the cardboard boxes.
Open the door and take the stairway on your right. Fight the zombies and press the generator button located between the two generators. Climb the ladder to enter the first courtyard.
hint: There is a breakable grate that leads into the water control ducts behind the service building. If the climb your way through you don't have to power up the generator or open the locked gate. You will also find a sub machine gun and a health pack along with two turrets.
Cross the court and look for a red button. It will open the locked gates. Enter the service building and make a left. Fight your way through the first of the combine soldiers and take the left or right path into the main courtyard.
Let the soldiers and antlions fight it out to reduce your ammo output. Kill whomever is left alive in the courtyard. Walk along the left wall all the way across the large court until you reach the far wall. Make a right and follow the wall on your left until you reach the grassy pit. There you can make another left and follow the ridge along the pit. Drop down onto the pipe the crosses the pit. Walk along the top of the pipe until you reach the platform marked with lambda symbol. There you will find a crossbow!
Drop into the pit and kill any headcrabs that fuck with you. Walk into the hallway that divides the two pits. Make a left and bust the grate in front of you. Fight your way through the basement and look for a ladder in the back right corner of the rock floor basement area.
Climb the ladder and kill everyone upstairs in the control room. Hit both the buttons on the security console. Exit the security room and cross the metal bridge back into the main courtyard.
Enter the left pipe in the first courtyard area.
hint : Once opened the right pipe will give you some hand grenades.
Walk down the pipe and make a right. Kill the zombies and soldiers at found at the bottom of the stairs. Look out for flanking combine.
Make a right when the hallway splits and enter the dark red hallway. Run as fast as you can down the hall to avoid the giant antlion busting through the walls behind you. Run across the blood spot on the ground and pick up some SMG grenades. Make a right and hide behind the dividing wall in the red lit area. Use SMG grenades and crossbow bolts to take out the huge alien.
Once everyone is dead cross the broken concrete ground and enter the control room. Hit the button under the security monitor to unlock the vault. Exit this area and watch out for flanking combine.
Head down the white hallway and make a right into the vault room. Look out for the turret on your left! Hit the hanging button to open the vault. Toss some grenades in and attack the combine guards. Look out for another turret on your left!
Continue down the hall taking out soldiers until the floor grating breaks under your feet. Walk down the next hallway and you will hit a level change.
Walk down the hallway until you can make a right. Use the health charger mounted here and ignore the floating camera. Then enter the next room and kill all the combine!
Go through the door on your right and down the stairs. Shoot the poison zombie and wait for the door on your right to open. Fight your way down into the mines and try to get to the lowest level. You will find a gravity gun on the lowest level.
hint : If you find yourself on the rock floor of the mine room look for a red light with a vent underneath it. Bust your way through and fight some antlions in the dark. Then you will find a hidden rocket launcher.
Once you have the gravity gun you can return to the first room of this map. Exit the mine and go back up the stairs. This time take the only other doorway available from the first area. This will take you into the generator room. There are two large ladders outside the generator room that lead up to the maintenance area. Climb one of the ladders and walk out onto the plat form above the generator.
There is one battery already on the platform and four above you held in their chargers. Use your gravity gun to pick up the batteries and place them into the four holes spaced across the platform. Once all four slots are filled the generator will be running on full power.
Climb down into the generator room and press the center button under the color display of the robot. The will recharge the robot and allow you access to its holding station.
Exit the generator room and enter on of the three vents located in the next room. Crawl around inside the vents until you find a path leading down. Take this duct into the storage room on the lower floor. You will find another crossbow here. Hit the button to open the security gate.
Walk down the platform and make a left to exit the room. Continue going straight until you have to make a right. This will lead you into the detention room. Ingore the prisoners here.
Make a left and go up a short set of stairs. Kill the combine in the next room and open the glowy blue force field door. Continue through the hallway until you hit the level change.
Ride the elevator to the top floor. Kill the zombies and soldiers that greet you and charge up your health and armor in the room where you found the zombies. Cross the hangar room and stock up on ammo in the opposite room. Be careful crossing the walkway as you will be taking a lot of fire from combine soldiers and a strider guarding the hangar gate.
hint : If you can kill the strider using the rocket ammo box you will save yourself some trouble down the road. Its a difficult shot, but killing him before you board the robot will enable you to fire all the robot's armaments while taking little flak.
Use the smaller elevator to enter the robot. The center button located in his head will engage his autopilot.
hint : There are two health chargers and two armor chargers in the lowest level of the robot's torso. Use these to keep you alive for the duration of the robot ride.
Once the robot is moving use his mounted guns to fight off combine attackers. There are two energy mortars mounted in his heads. You will also find buttons that fire his right and left arm cannons. Inside the robot's torso you will find a rocket launcher on the front and one in the back. These are very powerful rapid fire rocket launchers.
Once you get halfway down the hanagar the main gate will start to close. Once its fully closed concentrate arm cannon or rocket launcher fire on the doors to bust them open. Failing to bust open the doors will result in death.
Outside the hanagar you will have to face a combine helicopter. Use the mounted rocket launcher to take him out.
Before you can escape the compound an enemy controlled robot will lift out of the ground to face you. At this point you should climb into your robot's head to avoid the enemy arm cannons. Fire your energy mortars to take out the enemy soldiers. Use your arm cannons to bust out the other robot's armor plates and eventually you can knock his arms off. Once he has been neutralized focus your fire on his head and take him out.