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).
Construction No details of the Milecastle's construction have been identified.
Excavations and investigations 1966 – English Heritage Field Investigation. It was noted that there were no visible remains, and that the area was greatly disturbed by quarry tracks and spoil. 1989 – English Heritage Field Investigation. It was again noted that there were no visible remains, and the probable location was restated.
Associated turrets 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 Turret 21A (Red House) is probably located to the east of the east wall of Onnum, though some doubt has been expressed by an English Heritage field investigation. Its remains lie under the modern road.
Location on Ordnance Survey 1:25 000 map:
Turret 21B 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.
Monument records Monument Monument Number English Heritage Archive Number Milecastle 2120473NZ 06 NW 10 Turret 21A18154NY 96 NE 1 Turret 21B18160NY 96 NE 3
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