Approaching the blue key lowers a part of the walkway with two health bonuses on it in the south of this area, right west of the blue door, allowing you to access a hidden teleporter. Quickly enter it before it closes to get to the eastern balcony with the green armor. Leaving the balcony will teleport a pain elemental to the center of this area.
Go through the yellow door, up the stairs, and through the left door. Fall forward one level into a blood pit, and turn right (south). Open the wall to reveal a rocket launcher and two rockets.