Built in the 1530s, Craignethan is one of the first fortifications in Scotland purpose-built to withstand artillery. The impressive tower house was surrounded by a curtain wall on three sides, with a thick rampart on the other fronted by a ditch protected by a caponier. The defence device, which allowed defenders to fire across the ditch while remaining under cover, is unique in Scotland. However, it proved ineffective as a lack of ventilation caused the confined tunnel to quickly fill up with smoke when the guns were fired.