Milefortlet 21 (Swarthy Hill) was a Milefortlet of the Roman Cumbrian Coast defences. These defences were contemporary with defensive structures on Hadrian's Wall. Its remains exist as reconstructed turf ramparts (to a height of about 1 metre (3.3 ft)) and ditch, reinstated to its original profile (also around 1 metre (3.3 ft)). They are located on a low cliff on the coast, north of the village of Crosscanonby, and just inland from the coastal road.