| Milecastle 21 | |
| Location: | Northumberland, England, UK |
| Pushpin Map: | United Kingdom Northumberland |
| Pushpin Map Caption: | Location in Northumberland |
| Pushpin Label Position: | left |
Milecastle 21 (Down Hill) was a milecastle of the Roman Hadrian's Wall. The site was identified by measurement only, as the milecastle's remains have been totally removed. This is probably due to extensive quarrying at the site, which now lies under pasture (and partly beneath the Military Road).
No details of the Milecastle's construction have been identified.
Each milecastle on Hadrian's Wall had two associated turret structures. These turrets were positioned approximately one-third and two-thirds of a Roman mile to the west of the Milecastle, and would probably have been manned by part of the milecastle's garrison. The turrets associated with Milecastle 21 are known as Turret 21A and Turret 21B.
Turret 21A (Red House) is probably located 73m (240feet) to the east of the east wall of Onnum, though some doubt has been expressed by an English Heritage field investigation.[2] Its remains lie under the modern road.
Location on Ordnance Survey 1:25 000 map: 55.0109°N -2.0031°W
Turret 21B (Fence Burn) is probably located partly beneath the military road, though the exact location has been speculated due to the presence of a small mound along with dark soil and pottery discovered in 1930.[3]
| Monument | Monument Number | English Heritage Archive Number | |
|---|---|---|---|
| Milecastle 21 | 20473 | NZ 06 NW 10 | |
| Turret 21A | 18154 | NY 96 NE 1 | |
| Turret 21B | 18160 | NY 96 NE 3 |