Its main purpose was to guard the Caw Gap where the Haltwhistle Burn crosses the Wall.

[c] If so then the name might be interpreted as: Æsus is known to have been associated with water and river systems; the name might have been related to a remarkable aqueduct system that drew water from the Haltwhistle Burn – Caw Burn directly into the fort.

The fort had three main gates; south, east and west, with double portals with towers.

A barrack block was found and headquarters building (principia), together with its vaulted underground strong room.

The west tower of the south gate was found to contain a hoard of jewellery, which included an enamelled brooch shaped as a hare, a gilded bronze brooch described as a masterpiece of Celtic art, a silver collar with a pendant, a gold ring and a bronze ring with a Gnostic gem.