Grimrock

So, I got into this level by falling down the proper pit, found the Icefall ammer, then the wall falls away and there are some creatures to kill. But is that all there is to this level? Or should I be going to level 12 (I have the prison key) and at some point I am supposed to return to level 11?

Yes. you will return to level 11 from level 12. You can't miss it. It is part of the main quest.