Milecastle 51, one of the milecastle

Milecastle

A milecastle was a small fort , 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 , hence the name.Along Hadrian's Wall, milecastles were...

s on Hadrian's Wall

Hadrian's Wall

Hadrian's Wall was a defensive fortification in Roman Britain. Begun in AD 122, during the rule of emperor Hadrian, it was the first of two fortifications built across Great Britain, the second being the Antonine Wall, lesser known of the two because its physical remains are less evident today.The...

, is situated to the west of Birdoswald fort

Castra

The Latin word castra, with its singular castrum, was used by the ancient Romans to mean buildings or plots of land reserved to or constructed for use as a military defensive position. The word appears in both Oscan and Umbrian as well as in Latin. It may have descended from Indo-European to Italic...

 (OS grid reference

British national grid reference system

The Ordnance Survey National Grid reference system is a system of geographic grid references used in Great Britain, different from using latitude and longitude....

 NY 592 654), at the point where the Stone Wall

Hadrian's Wall

Hadrian's Wall was a defensive fortification in Roman Britain. Begun in AD 122, during the rule of emperor Hadrian, it was the first of two fortifications built across Great Britain, the second being the Antonine Wall, lesser known of the two because its physical remains are less evident today.The...

 rejoins the line of the Turf Wall

Hadrian's Wall

Hadrian's Wall was a defensive fortification in Roman Britain. Begun in AD 122, during the rule of emperor Hadrian, it was the first of two fortifications built across Great Britain, the second being the Antonine Wall, lesser known of the two because its physical remains are less evident today.The...

.

Although not now visible, excavation showed that it has two internal stone barracks

Barracks

Barracks are specialised buildings for permanent military accommodation; the word may apply to separate housing blocks or to complete complexes. Their main object is to separate soldiers from the civilian population and reinforce discipline, training and esprit de corps. They were sometimes called...

 buildings. It has Type III gateways. It lies 1501m west of Milecastle 50

Milecastle 50

Milecastle 50 , one of the milecastles on Hadrian's Wall, is situated to the west of Birdoswald fort ....

 and 1484m east of Milecastle 52

Milecastle 52

Milecastle 52 , one of the milecastles on Hadrian's Wall, is situated to the west of Birdoswald fort ....

.

The stone milecastle replaced its underlying predecessor, Milecastle 51TW on the Turf Wall.

Milecastle 51 was excavated in 1895.