There are few things that instill as much dread as seeing a steady stream of enemy Minions pouring into your dungeon, marching steadily down your hallways intent on inflicting as much destruction as possible. One of those "few things", however, is a handy little thing we call the Boulder Trap!
Unlocked and available to be built once your Workshop attains Level 4, the Boulder Trap causes a very large and very heavy boulder to roll down your hallways - flattening everything in its path, dealing significant damage to all targets thus steamrolled - once an enemy Minion gets within 5 tiles. The boulder will continue rolling in a cardinal N/S/E/W direction until it hits a wall, making it especially effective in long hallways where you're able to funnel large groups of attackers together.
The Boulder Trap is consumable, available for a single use before needing to be replaced. These traps are also NOT upgradeable. As a result, it has a relatively inexpensive cost, and is built instantly upon the build order being issued. No waiting!
Rolling stones might not gather much moss, Keepers... but you'll be amazed at the things that THIS rolling stone will pick up along the way. If you're squeamish, though... maybe DON'T look!