Grimrock

Durkam's castle (prison), Palace gardens, Forest of Arcane Academy, Eastern beach, Mine Level 1, Academy tower, Ancient water shrine, Durkam's tomb, Inner sanctum, Mine Level 2, Mine Level 3, Mine Level 4 (Eternal night)

I killed the big ratling in the mine so I used one mine key there but still have another one left

create the magic bridge. I did not find any hint for it and tried so many things until I put a power gem into one alcove and it opened the bridge. I find it weird that there are two entrances to the castle prison - through the water and through a teleporter from durkam's tomb and there is only one big corridor there (aside from three small rooms) and no hint what to do. Who puts power gems into alcoves just to see if that changes anything?

I wonder if that is all there is to the castle or if there is more to it. I had the same feeling in the mines when you go up the other entrance from the armor and weaponsmiths into a temple-like area. It was just a few rooms with no monsters and nothing of interest in there.

There is a wall text explaining to use a power gem to activate the bridge, but the wall text was on the wrong side of the wall, so you couldn't see it.