3 Defend This!

Tips

Many people get stuck in the first part of the level. They enter a five sided room, which is an elevator that goes up and down only once, but there is no other exits from it. Actually the exit is secret: in the left side of the wall across from the sole enterance there is a secret door. However, it is slightly off the ground, such that you have to ride the elevator a bit to continue on.

After this room, the corridor splits in two. The left branch takes you to a save terminal, a shield generator, and ammo, so it should be your first stop. The right branch continues into the rest of the level.

In the room with the 4 switches and the blue and white striped door, the best way I found for getting past the door is this: while facing away from the door, hit the rightmost switch, which temporarily opens the leftmost panel of the door (if facing the door). Get yourself 'wedged' in that space, and turn back to the switches, and without moving, launch a grenade at the second one from the right. Turn around, and just hit the keys for running forward. After a second, the panel to the right will drop, and you should make it through the door. Alternatively, you can hit the 2nd switch from the left, then hit the rightmost switch, followed by the 2nd rightmost switch. The opening should be large enough to run through. To leave, you'll need to grenade activate the 2nd rightmost switch again.

In the room where there are 4 switches and a window looking out onto a set of platforms, you only need to hit the 2nd switch on the left (when facing the switches) to get the platforms in the right configuration.

Secrets

There are two rooms that have a bright red band running in the center of the walls. In the second room, across from on of the doors, is a secret door leading to some ammo and a terminal.

In the room with the bridge, you can grenade jump to first get on the sides of the bridge (without falling off), then grenade jump again to get into the corridors (a backwards grenade jump in this case). All 3 corridors are connected, and lead to a stash of ammo.