It appears in the Domesday Book as Loncastre, where "Lon" refers to the River Lune, with the Old English cæster (borrowed from the Latin castrum) for "fort".

[4] The fort was reoccupied again during the 2nd century and it was rebuilt in stone around 102 AD, with an almost 2-metre (6 ft 7 in) thick revetment wall built in front of the clay-and-turf rampart.

On the northern side, an approximately 3 metres (9.8 ft) thick wall with polygonal external bastions, was constructed in a south westerly direction, over the remains of the earlier forts.

[13] It is thought to be the core of the external bastion at the northern angle of the wall's circuit and was likely topped with a heavy artillery piece.

In the 18th century, a fragment of the south wall was recorded, noticeably not parallel to the north, but thought to belong this fort, suggesting it did not follow the classic Roman plan.

[4] Next to the Wery Wall (near the junction of Bridge Lane and Church Street), the remains of a bath-house incorporating a reused inscription of the Gallic Emperor Postumus, dating from 262–266 AD, were discovered in 1812.

[18] In March 2023, students from the Lancaster University announced the discovery of a Romano-Celtic religious temple near the castle during a hydrogeophysics training session.