It was built on the Stanegate frontier and Roman road, linking Coria (Corbridge) in the east to Luguvalium (Carlisle) in the west, before the building of Hadrian's Wall.

[2] The name is rather inappropriate for a relatively small fort,[5] and one suggestion is that it could ultimately derive from the Celtic word maen meaning "stone" or "rock".

[6][7] The site was occupied by several earlier timber-built camps as revealed by aerial photography, and a large enclosure of just under 8 acres (c. 3.2ha) below the later walls.

Further rebuilding was done at the end of the reign of Antoninus Pius (c. 161) and after the withdrawal from the Antonine wall, the same garrison that had occupied it in Hadrianic times fort returned.

Artifacts recovered at Magnis include a 2-foot-long (0.61 m) iron spearhead, found at a depth of 36 feet (11 m) in a well, and the well-known modius, a bronze grain-measure.

A large gallery describes daily life in the Roman army as seen through the eyes of a team of eight auxiliary soldiers, complete with a film showing their activities.