Its still used today in some parts of the world.
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 25-30 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.