Half-Life/Lambda Core

The Reactor
On the other side of the large door, you will find a truck sitting on a lift. Activate the lift by hitting the button in the booth. Wait for the lift to start descending, then jump on top of the truck. At the bottom, there are several headcrabs scattered among breakable boxes and a bullsquid in a tunnel around the corner. Break the boxes for supplies, then go through the door and climb down the ladder. Downstairs, for the second time in the game, you will run into assassins. The next room is somewhat open, filled with stacks of green boxes and some small explosive boxes and barrels. There are also catwalks above that assassins will constantly use to pick at you from above. Equip the magnum or shotgun before entering this room, exploring it cautiously, killing any assassins that you encounter. Remember that snarks work rather well against assassins, as do weapons that deliver a lot of damage in one blow. On one side of the room, there are several breakable boxes blocking a doorway. This leads up to the catwalks above where more assassins may be lurking. On the opposite side of the room is another stairway leading down to a health station and an HEV station. When you've recharged, go back up to the top and look for an elevator somewhere in the alcoves here and take it down.

At the bottom, you will be in a large, semi-circular tunnel. As you approach the door, an alien grunt forces it open and an alien grunt teleports in behind you. Give the alien in front you a good shotgun blast or two to the chest, then switch to the pistol, turn around, and shoot the explosive barrels by the alien behind you. Two more alien grunts will teleport in and you must kill them to proceed. When the last alien grunt is dead, a scientist will unlock the door for you. He will tell you that you need to restart the reactor and flood the core to get to the teleportation labs. Gather the ammunition and use the health and HEV stations here before taking the lift up.

Go down the hallway and take a right at the fork to find a scientist an interesting weapon. Save the game if you want to test it out without wasting ammo. Otherwise, just take the weapon and ammo and leave. If you choose, open the gates to the alien's cells and leave the scientist to his fate. Go back to the fork and go the other direction. This hallway will take you to the lift that leads to the nuclear reactor's core.

The Core
At the bottom of the lift, you will find yourself on a balcony overlooking pools of water. Here, unlike just about everywhere else in this place, the areas are clearly marked. Thank goodness. Follow the blue signs and stripes that say "Coolant System 01". Just before the door to the pump room, a vortigaunt will teleport in behind you, so be wary. The signs will take you to the pump station, with a large pool in the center, guarded by alien grunts. When they're down, enter the room and climb up a ladder. A catwalk up here leads to the pump room infested with headcrabs and vortigaunts. Kill them using whatever means you please, but I think you'll find that your recently acquired Hivehand is curiously efficient at clearing headcrabs. When you've cleared the room, find and flip the switch marked "Pump 01". This will cause the reservoir to fill with water as well as several more aliens to teleport in. Clear them out, then leave the room.

Follow the blue signs back to the lift, then follow the orangey yellow signs leading to "Pump 02". The hallway to the second pump will collapse as you go down it, and may seem blocked off, but if you approach the rubble, you'll find that there's still a way through. This hallway leads to a pool guarded by alien grunts very similar to the first one except that there are barnacles on the ceiling here. As you walk on the upper catwalk, an alien grunt will teleport in and break it in half. Back up as he teleports in and drop a grenade or two on his head. Further on, there is a steam leak blocking the path. Turn the valve wheel to your right to stop it and crawl through the gap. Down the corridor and up the stairs, you will find another pump room infested with headcrabs. Drop down and pull the lever to flood pools. When you've pulled the lever, check the chart on the wall. It should show that both Pump 01 and Pump 02 are operational.