The remains of Milecastle 42
Milecastle 42 is situated on a steep south facing slope, 10 metres south of Cawfield Crags, and looks over Hole Gap to the west. It is on a well preserved section of Hadrian's Wall. It measures 17.8 metres east-west by 14.4 metres north-south internally, with walls 2.8 metres thick and 1.4 metres high.
It was excavated in 1847-8 and again in 1936. The 1847-8 excavation uncovered part of a dedication slab indicating that the milecastle had been built by the Legio II Augusta. The 1936 excavations showed that the milecastle had a short axis, with a Type I gateway. 
Each milecastle on Hadrian's Wall had two associated turret structures. 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. The turrets associated with Milecastle 42 are known as Turret 42A and Turret 42B.
The milecastle and the site of Turrets 42B are both accessible via the Hadrian's Wall Path. Cawfields Picnic Area car park is on the line of the Hadrian's Wall Path, between the two sites. It is signposted from the B6318 (Military Road).
Media related to Milecastle 42 at Wikimedia Commons