At the start of the level there is a guy who can give you the gun. You need to collect pies for ammo. You cannot pass the canal until you go inside the building to set the bridge down. The second bridge you get to you go inside the building , but this time you go up inside the building to the rooftop. From the rooftop you can jump across to the top of the bridge supports, where there will be a boss monster. Bowser style, you avoid the things he throws at you and attack until he falls off the bridge, into the water. You can jump down off the other side to another rooftop. going back to ground you eventually get to the yellow onion.
When you enter the room, it is dark, except for zombie noises, maybe glowing eyes. There is also a glowing light switch, which you activate.
You need to kill zombies and get up to the switch to activate the bridge machine, which lowers the bridge.
In the second room you will just climb up to the rooftop, as the machine will be broken. This leads you to the boss monster.