This is the forum for helping you out if you're stuck or if you want to discuss the nitty gritty details of the dungeons within Mount Grimrock. Warning: forum contains spoilers!
You open the doors in lvl 12 with key you find in lvl 12. As soon as "Da Boss" appears and all the creatures of doom spawn, the keys appear "somehow"; I haven't figured this exactly - either they appear right after the boss appears or they appear randomly depending on spawned monsters being killed.
You recognize the keys lying on the floor; and if you get near to them they "blink" or "shine" for a tenth of a second or something ...
All the best!
Liam
At the end of my first time with "the Legend of Grimrock" - played on default-party, hard, old-school mode: