(All pictures below by Ken Fitzpatrick)

Milecastles.. A milecastle was a small fort (fortlet), a rectangular fortification built during the period of the Roman Empire. They were placed at intervals of approximately one Roman mile along several major frontiers, for example Hadrian's Wall in Great Britain (Britannia in the Roman period), hence the name.

Along Hadrian's Wall, milecastles were initially constructed of stone in the eastern two thirds, and stacked turf with a wooden palisade in the western third, though the turf milecastles were later rebuilt in stone. Size varied, but in general they were about 15m by 18m (16 by 20 yards) internally, with stone walls as much as 3m (10 feet) thick and probably 5m to 6m (17 to 20 feet) high, to match the height of the adjacent wall. There were 80 milecastles and 158 turrets.

are numbered from 1 (the easternmost Milecastle) to 80 in the West. This system was introduced by
J. Collingwood Bruce at the end of the 19th century, and became a standard around 1930.

80 Milecastles
were built along Hardian's Wall.

Here i shall only show 8 of the Milecastles along Hadrian's Wall these eight have visable ruins and can
be visited...
Number 33... Shield-on-the-Wall.
Number 35... Sewing Shields.
Number 37... Housesteads... These remains just west of Hosesteads Roman Fort have been partly reconstructed
and consolidated; it is now in the care of English Heritage. The wall has a maximum height of 2.2 metres internally. 

The milecastle has a short axis, with a Type I gateway. The milecastle contains the remains of a small barrack block in the east half which survives to 1.0 metres high.

The first excavations of Milecastle 37 were in 1853, 1907 and 1933. A re-used corner of a Hadrianic dedication slab was found in 1853, and two altars inscribed to Cocidius and Jupiter were found near the milecastle. Excavations in 1988-9 showed three periods of the north gate, having been built, blocked, then partly demolished.

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 37 are known as Turret 37A and Turret 37B.

Roman Wall 
Roman Wall 
North gate of the Milecastle 37 west of Housesteads.
Number 39... Castle Nick.
Number 42... Cawfields.. 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 1847–48 and again in 1936. The 1847–48 excavation uncovered part of a dedication slab indicating that the milecastle had been built by the Legio II Augusta.

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).
Cawfields, Milecastle 42
Cawfields, Milecastle 42 
Number 48... Poltross Burn.  Number 49... Harrows Scar.  Number 53... Banks Burn.

From wikipeadia