Ki and Gil will enter this room through the door down at the bottom-left of it. It contains a total six Red Slimes, six Blue Slimes and six Green Oozes.
The key at the bottom-left of the room opens the aforementioned door at the bottom-left of the room, and the key at the top-right of the room opens the door at the top-right of the room. There is no key for the door at the right of the room, so there is no way of knowing which room it leads to now.
The aforementioned door down at the bottom-left of the room leads back to Empty Cistern (Room Pass: H32), and the door up at the top-right of the room leads to Colosseum (Room Pass: HC2). You should remember the second of these Room Passes - as the first of them is little use to you now.
