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. The earliest dateable evidence is from the end of
Domitian's reign (1st century). Magnis was built on the
Stanegate frontier in about 80 AD to guard the junction of the
Maiden Way Roman road (running north to south) with the Stanegate (running west to east). As such the fort predates
Hadrian's Wall. The
Maiden Way ran south from Magnis to
Bravoniacum (
Kirkby Thore near
Penrith). An intermediate fort halfway between the two on the Maiden Way was
Whitley Castle or Epiacum, just north of
Alston, Cumbria. Five hundred
Hamian archers, known as
Cohors Prima Hamiorum Sagittaria, the only regiment of archers known in Britain, were stationed at Magnis starting from 120 AD. Hadrian's Wall was built from 122 only a few hundred yards north of the fort, and its
Vallum ditch was dug to the north of the fort, separating it from the Wall and the frontier zone. The vallum was also diverted much further north of the fort than needed with no apparent reason. The fort was rebuilt in stone in 136-7 when the
Antonine Wall was built in what is now Scotland. 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. ==Vicus==