Note: in order to reach this level you must pull the switch in the Sacred Grove (Episode 2). If you don't pull that switch, the portal to this level won't open. Also, some people have mailed me saying that the portal doesn't open even though they pulled the switch on the Sacred Grove. I'm not very sure about this because so far I haven't encountered the same problem. But using the VISIT cheat code or the CASPER cheat code may cause you to miss certain trigger lines on the levels, and mess up the script that opens this portal. This could be a possible cause of the problem.
The portal to this level is in the Forsaken Outpost.
OK, jokes aside. If you look at the ground, you'll notice eight switches around the garden. These pop out of the ground randomly and pop back in. You need to catch each of the eight switches when they pop out and pull them. Well, you may think this is a funny game, but the switches don't think so, so be prepared as some monster teleports in each time you pull a switch...
After all eight switches are pulled, the elevators on the north and south sides of the garden are activated, and the stairs there are opened as well, leading to new places to explore.
If you look carefully, there are drainholes on the floor in this other room, and there's a pillar on the far side of the room. One of the three parts of your fourth weapon is here too. Uh-oh! muck is overflowing from the pillar into the room! The water level (or muck level) is getting higher and higher... the ceiling is getting closer and closer... Hurry up and run behind the pillar at the far end of the room. Activate the switch there. This switch stops the overflowing muck coming from the pillar, and the muck in the room will drain away through the drainholes on the floor.
That's not all, though. A jealous Chaos Serpent will appear in a newly opened area opposite the door of the room. One of the parts of your fourth weapon is inside.
If you go to both the two sections, a hidden area will open, joining the two sections, as you leave. There's a Krater of Might inside.
That's about all you can do in this level. Grab all the goodies and items lying around, and step into the portal to the Forsaken Outpost to continue your quest.