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
