Grimrock

Ok... I'm on the Prison level. I've killed the guardians, the dementor wannabes and everything else I could find.

I see multiple doors that require keys - but I have no keys.

I've been down in the pits, collected a bunch of stuff but I have no idea what to do with it.

Please... any help would be appreciated!

On Floor 13 you should find a Bladed Gear, a conductor or something, a "small gear" (look at the description, it'll say its small) and a piece of ore. Once you have those 4 items you insert one into each side of the broken pillar of machinery in the center of floor 12. The items might not be easy to insert, you have to click all over the broken parts of the pillar to find where the parts belong.

BE WARNED. When you insert the last part you reach the "point of no return" and you cannot return to older levels after that, you may wish to make a special save just in case you want to go back and explore the earlier floors.