| Milefortlet 21 | |
|---|---|
 Milefortlet 21, with the post-medieval Allonby Saltpans on the seaward side | |
  Location within Cumbria | |
| Type | Milecastle |
| Location | |
| Coordinates | 54°44′48″N 3°27′01″W / 54.746648°N 3.450398°W |
| County | Cumbria |
| Country | England |
| Reference | |
| UK-OSNG reference | NY06734004 |
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. It is the only milefortlet to have been fully excavated[1] and is open to the public.
- ^ Breeze, David J (1934), Handbook to the Roman Wall (14th Revised edition - Nov 2006), Society of Antiquaries of Newcastle upon Tyne, pp. 394–395, ISBN 0-901082-65-1