The first thing you see when Foehammer drops you off is a Grunt running into a door. Quickly run to the nearest gun turret and get in. As soon as the door opens, start blasting the Covenant that come out. There's an Elite in there as well, so watch yourself. If they get out of your range of fire, get out and finish them off with your Assault Rife or Pistol. After killing them all, grab any plasma weapons that you fancy (if you fancy them that is) and head for the door. Be careful, there are still some Grunts inside. Kill them when you run into them. Walk quickly along the hall way. When you get to the next door, be careful. There's an Elite outside. Take him down any way you want. As soon as he hits the floor, continue onward to the right. There are quite a few Jackals and some Grunts in here as well. Don't kill all of them (unless you are on a higher difficulty), you'll need your ammo later. Move around the place until you come to another door. Look on the ground in front of it. If it has a glowing arrow-like thing on pointing towards it, go in. If its pointing away from it, don't enter. This is the door you came from. When you find the right door, head in. Be careful of the Grunts in here. Continue onward once more. After several more twists and turns, you will come to another door. This will lead out to a bridge. Now comes the fun part. There are several Grunts asleep here, so melee them to death. If you screw up and wake them up, kill them normally. Continue carefully across the bridge, as there are more enemies here.
If I Had a Super Weapon...[edit]
In the final section, you will reach a natural bridge with covenant patrols and two banshees, and also a wraith that tries shelling you from the ground below. Against the enemies on the bridge, you may want a homing weapon (such as a needler from the previous enemies). The banshees are a bit more of a problem if they take off, but can still be shot down if you move in and out of the doorway (unless you quickly capture one of the vehicles.) The middle of the bridge has three covenant enemies. When you approach the door, you may want a heavy weapon or explosives, which you can fire upon the two hunters that try exiting the door.
If you proceed through the door, you will need to naviagate a few more rooms. The first room has several weak enemies, and requires circling around the left until you reach a ramp heading down. This room's exit leads to an elevator bringing you down to a second room, where you circle around counterclockwise (and the enemies aren't paying much attention. The second room's exit leads outside.
At ground level, there are a few ghosts you can use. The first is to your immediate left and should be grabbed before the enemy decides to use it. Other ghosts can be found at the far corner. With the ghost, ascend the pyramid structure. There will be enemies at each level, including a pair of hunters at the base, and a pair of hunters when you reach the door. There's nothing in the structure going further above.
When you are ready, open the door. This will release a large wave of grunts, and one elite equipped with a sword. You can use either a turret or a full-health ghost to gun down the enemies as they approach, although you should abandon the turret as soon as an enemy attempts to throw a grenade. Once enemies are down to a manageable level, enter the doorway. The last few stragglers are around the corner. Open the last doors to reach the control room.
In the following cutscene, Cortana installs herself into the Halo control room, and determines there's something wrong with the cache that Captain Keyes is trying to open.