A fortlet (small fort) set at intervals of one Roman mile along a major well-defended frontier - such as Hadrian's Wall. They measured 60-70 x 50-60 ft. A roadway passed through the center of the milecastle and through the wall itself, with gateways at both entrances. There were two barrack buildings parallel with the road. In one corner, next to the wall, was a stairway up to the wall top; in the other was the cookhouse. It is estimated that 30-100 men could be accommodated in each milecastle.