Your first goal in old town is to possess a commander. If you're still in the body of a commander, or you have the bazooka, this won't be necessary. If neither of those conditions apply, the first order of business is to get your hands on a pump gun.

As you exit the shipping sector, there are a few riot cops near a barrier to the right and commander in a shielded post near a barrier to the left. There are some pump guns to the right, so head that way and pick one up. If you're in need of a host, possess a riot cop and run to the other end of the street. A chot invasion will begin, and you can then possess a chot.

When you have a host and a pump gun, shoot the explosive crate near the commander's post. This will destroy the barrier and release the commander. Possess her.

Once you have possessed the commander, get on the small elevator that leads up to the roof. On the roof, you'll see a console. Use the console to activate the gun turret. Use the gun turret to dispose of any chots on the street level, then shoot the remaining barrier. (If you brought the bazooka from the shipping sector, you can just shoot the barrier when you enter old town and ignore everything else.)

Go back to the street level and run up the stairs near the newly destroyed barrier. It will be helpful to have a chot pak weapon for the next section, so pick one up before you enter the recycling facility.

In the recycling facility, run forward until you are facing the scientists behind the laser barrier. You must get up there. Drop your weapon, then jump up onto the small ledge to your left. Once there, depossess your current host, so that Bob lands on the ledge. Crouch and crawl under the lasers.

Possess the scientist behind the console, then activate it. This will deactivate the floor, and the other scientist will fall into the recycler. Another scientist will appear. Repeat this process until the recycler is full. Depossess the scientist and crawl back under the lasers. Glide to the floor.

Possess the worker. Activate the console to the right of the recycler. Pick up the pak and follow the barrel up to the raised platform.

Climb onto the barrel, then jump to the ledge next to the incinerator door. The flight ability of the pak will be useful here, and you can activate it by holding down the jump key. If there's no pak weapon ability, use Bob's ability to fly to reach the ledge. On the ledge is the console that will open the incinerator door. Use the console, then descend to the floor. The door is only open for a few seconds, so quickly run into the incinerator.

Climb up the barrels, then jump to the ledge. Use the pak's flight ability to access the smaller ledge above. There is a small pipe, which is on the left if you are facing the incinerator. Jump to this pipe, then head up to the wall above.

You'll be looking down on a small science outpost, manned by some military personnel. Use the pak to safely drop to the floor. There's an elevator in the center of the room. Ride it to the second level, then open the door and enter the small closet.

Depossess the worker, then possess one of the rats scurrying around the closet. Run into the vent and follow it down. When you exit the vent, you'll be in a basement filled with waste and human bones. You must get to a second vent on the other side of the room. Stay in the rat and walk over the raised areas, using the bones as bridges when applicable. The waste level will rise and fall, so time your movement accordingly.

Follow the second vent up until you emerge on a small ledge. Drop down and possess the chot in the room below. Activate the detonator on the box, then exit through the air vent. Drop down to the floor, where you will see another vent. Make sure you aren't in combat mode, then prepare to enter the outskirts.