Turret 39A (Peel Crag) (grid reference NY75626763) was located in 1909 and excavated in 1911.

[3] In summer 1987 excavations along Hadrian's Wall (made jointly by the National Trust and English Heritage) discovered the foundations of a Roman tower between turrets 39A and 39B.

The tower is located 345 metres (377 yd) west of Turret 39A (grid reference NY75336748) in the lowest part of Peel Gap.

The doorway (located at the east end of the south wall) was found to have been blocked up.

[4] The milecastle, the sites of both turrets, and the remains of Peel Gap Tower are all accessible via the Hadrian's Wall Path.