“Bowling alley” themed room, a fixed size (either 5 in width or height, longer in the other dimension) with six statues arranged in a triangle at one end of the room as “pins” and one or more rolling boulder traps for the “ball”.

If you trigger a rolling boulder trap for which a boulder didn’t generate because there was no room to put it, a boulder may fall out of the ceiling instead (and the trap is removed) similar to a cursed scroll of earth. On early levels, it doesn’t have to fall on the same square as the trap.

Also, rolling boulder traps (beyond early levels) that do find appropriate launch locations may generate with no preexisting boulder on one of those positions. When triggering the trap for the first time, a boulder drops out of the ceiling on one of the launch positions and then begins rolling across the trap.

The rumbling from a rolling boulder trap wakes up monsters.

Rolling boulder traps don’t expect a boulder at a specific spot: they look up to 9 squares in all 8 directions for boulders, and the first boulder it finds will be triggered. If no boulder is in range, it may create one by dropping it from the ceiling in a suitable location, then rolling it across the trap (but only one boulder per trap will be created like this, and some traps generate incapable of creating a new boulder).

Rolling boulder trap hit chance and damage is based on dexterity and not AC, for how much you can jump out of the way. Only high dexterity will allow you to dodge the boulder entirely.

Slightly increase the minimum depth of rolling boulder traps, which is currently 2 and can instakill a character on level 2 easily.