Desolate Garden is the fourth hub's secret map, available to those who completed the secret levels of the first two hubs.
Look for three superweapon pieces. One is behind the exit portal, one is in the eastern slime trap room, and the third is visible when you trigger and disarm the slime trap.
Switches[edit]
You will start in an open courtyard, with a 3-by-3 arrangement of switches that will rise and lower. There are also enemies attacking from the windows in the eastern and western walls.
You will need to pull switches to unlock parts of the level. When you pull a switch, one random monster will be summoned, except for the central switch which will summon a group of Dark Bishops.
After pulling four switches, you should have both the northwest and southwest elevators active. If you cross the upper threshold of both elevators, there is a delay of 14 seconds before a section of the western corridor opens, giving access to a Krater of Might.
After pulling six switches, you can access the stairs leading to the east of the map. There is a room containing slime, and entering it activates a trap. There is a short delay before the trap activates, and leaving the room makes the contents inaccessible. The trap seals you in the room, requiring you to activate a switch in the eastern end. When the switch is activated, the room will drain and you will see a superweapon piece just outside (along with a Green Chaos Serpent).
When you pull nine switches, the exit is available between the two stairs. Remember to circle around the exit portal to collect the last superweapon piece.
The exit returns you to the Forsaken Outpost.