Wikipedia
A milecastle was a small fort (fortlet), a rectangular fortification built during the period of the Roman Empire. They were placed at intervals of approximately one Roman mile along several major frontiers, for example Hadrian's Wall in Great Britain ( Britannia in the Roman period), hence the name.
Along Hadrian's Wall, milecastles were initially constructed of stone in the eastern two thirds, and stacked turf with a wooden palisade in the western third, though the turf milecastles were later rebuilt in stone. Size varied, but in general they were about 15m by 18m (16 by 20 yards) internally, with stone walls as much as 3m (10 feet) thick and probably 5m to 6m (17 to 20 feet) high, to match the height of the adjacent wall. There were 80 milecastles and 158 turrets.
On Hadrian's Wall, a milecastle (there are a few exceptions) guarded a gateway through the Wall with a corresponding causeway across the Wall ditch to the north, and had a garrison of perhaps 20–30 auxiliary soldiers housed in two barrack blocks. On either side of the milecastle was a stone tower (turret), located about one-third of a Roman mile (500m or 540 yards) away. It is assumed that the garrison also supplied soldiers to man the turrets. The milecastle's garrison controlled the passage of people, goods and livestock across the frontier, and it is likely that the milecastle acted as a customs post to levy taxation on that traffic.
A system of milecastles (known as milefortlets) and intervening stone watchtowers extended from the western end of Hadrian's Wall, along the Cumbrian coast as far as Tower 25B at Flimby, but they were linked by a wooden palisade and not a wall fronted by a deep ditch, and they had no gateway through the palisade.