Grimrock

I've come to a screeching halt early on, and I think I must have missed something fairly obvious.

I'm at the cells in the old tunnel, just after . I can't open any of the cells and I don't have a key for the door at the end. I've scoured the 2nd level, and I'm about to head back to the 1st even though I can't imagine the key would be up there. Where should I be looking?

not sure if i remember correctly, cells...