The scar or cliff and hence the milecastle are named after an ancient tenement called The Harrows which stood nearby.

Harrows Scar measures 19.8 metres east to west by 22.9 metres north to south and no contemporary internal buildings are now visible.

A. Richmond,[2] when parts of the underlying Turf Wall milecastle (49TW) were identified.

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.

Turret 49A (TW) (grid reference NY61546628) was constructed when the turf wall was built.