Ki and Gil enter this room through the door at the right of it. It contains five Orange Vampires and ten Purple Bats (both of which take four hits to kill).
The key at the bottom-left of the room opens the aforementioned door at the right of the room (there are no keys for the doors at the centre-left and top of the room). Therefore, there is no way of knowing which rooms those other two doors lead to - until you are led back to this room through one.
The aforementioned door at the right of the room leads back to Observatory (Room Pass: YC9). It would appear that the "DON'T ENTER BE GONE" sign in that room was there for a reason - because you will just have to go back out the way you came in, risking your life to collect the only key for it.
