That was certainly quite interesting. The first couple of levels explain the mechanics well. However, I couldn't figure out how to complete the level with the cats. I guess they need to fight each other, but I'm puzzled.
The objective is the same in every level. You just have to find the door. I'm guessing your talking about level 4 where the cats are first introduced. There is a door in the same room as one of the cats, the really big room. You just have to go in there.
