Hall of Knives
While it looks like a kind of aggressive space, with all of those sharp rocks pointing inwards it's actually just a storage room. These rooms were constructed on a frame, and then the builders stacked all the rocks over it, eventually jamming in the ones at the top from above until it was completely solid. After that the frame could be removed and you end up with this a room.
This one is in the Old Mellifont Abbey, County Louth.