This level is DIFFICULT. There are many big melees without a lot of cover to fight from, plus Roto-droids are in no short supply. These are those big muscleman-dudes who shoot darts, detonation mines and big floating energy orbs that--needless to say--do a lot of damage. They're easily the toughest non-boss enemies in the game. This level overall has the toughest combat on average in the game (have fun!).
Elongated rooms[edit]
The big elongated room with the cages is the first of three rooms like this, that are interconnected. There's a bridge leading to an overlooking room, and a large door near one end that's unpowered. Hit the switch at the other end of the room to power this door. At the end of the room near the door, activate the protruding section of wall with blue+green texture for a secret area.
Go through the large door to a big connecting corridor with red-waste channels on either side and a bridge overhead. Look for an irregular patch on the wall just to your left, and shoot at it to reveal a secret area with a Secret Level Access Keycard. (If you don't see it, just shoot at the wall in that area. You'll find it easily enough.) Continue to the second elongated room past the far end of the corridor. There's an unpowered door at each end of the room; you'll get to these later. Near one end of this room is a doorway; go through here to the third elongated room. To the right is an unpowered lift leading up to the bridge in the connecting corridor. Opposite the room from the lift is a door going to two small rooms in series that are jam-packed with Roto-droids. You gotta go in there, so... This is a good place for Death Spheres and Missile Hunter-Killers and other things you can set off inside and then leave. After you get this mess dealt with, hit the switch in the far room to power the lift.
Take the lift up and follow the corridor over the two bridges to the overlooking room by the first elongated room, where there is a row of chambers with enemies. Hit the switch to power one of the two doors in the second elongated room--and release the enemies from the chambers. When you've fought them off, shoot the back wall of the leftmost chamber for a secret area.
Glass cages[edit]
Go to the second elongated room to the powered door (the one nearest the doorway to the third elongated room), and follow the corridor with the wall emplacements to a big door at the end. Get your weapons ready, because you have a substantial melee waiting inside. This is extremely important: you must be inside this room when the door closes. At this point, the door can only be opened once. If you duck out and it closes while you're outside, you won't be able to get back in and will be stuck and unable to complete the level. (This appears to be an oversight in the level's design. A switch outside the door that opens it would have solved the problem.) After clearing out the enemies, hit the switch at the back of the room. Your primary objectives in this level are now completed. The door to this room will now function normally.
Go back to the second elongated room, through the door at the opposite end of the room. From the room in here is a second corridor going back to what was a hidden door in the first elongated room. (You'll notice part of the corridor is flagged "hidden" and won't show on the automap.) Along the corridor is a side area with the level exit. When you've finished catching your breath, and have seen the sights all you want, go to the exit elevator: after getting bashed up in this level, you'll get to experience more pleasant things. Some really fabulous level design is in store.