Des
I think I'm about to describe Castle Kriegspire. If not, sorry.
There are four tall rooms that have ledges running around the second
story level. The four rooms look kind of like a four leaf clover
on the map and the monsters seem to drop from the ceiling. (Though
I think they are really supposed to be jumping off the ledges.)
Anyway, get up on the ledge and go to the center of the four rooms.
There's a column in the very center. On the back of the column is
a switch. I walked around the column a couple of times and didn't
see the switch so, it can be easy to miss. This will open a door
that leads to the crystal.