A large rolling ball (imagine Indiana Jones) is released that knocks off the map any troops in its path (like a bowling ball knocking out pins) until it reaches the edge of the map and stops. The trap is one-directional like the Air Sweeper and also limited to two. It could look like a platform with a large boulder on top with a ramp attached to it. The down-side? Any of your walls in its path get destroyed and buildings lose hit points so be careful where you place it ;)
