The Anglo-Normans first arrived in Limerick in 1171 when Donal O’Brien, King of Limerick and Thomond, paid homage to Henry II at Cashel. However, relations quickly soured; by 1175, O’Brien had rebelled against English rule, prompting Raymond le Gros to march on the city with a formidable force of 120 men-at-arms, 300 mounted soldiers, and 400 archers. The castle itself appears to have been constructed in the early thirteenth century, possibly on the site of an earlier fort mentioned in the Cogadh Gaedhel re Gaillibh. By 1211-12, there was already a substantial structure requiring £733 16s. 11d. in repairs, indicating the castle had been standing for some years.

The fortress forms a rough quadrangle measuring 75 metres north to south by 65 metres east to west, originally featuring round towers at each corner and an impressive twin-towered gatehouse at the centre of the north wall. Built from coursed limestone rubble with alternating courses of large blocks and small pinnings, the castle has undergone considerable modification over the centuries. The gatehouse, which still serves as the main entrance, consists of two D-shaped towers flanking a central passage that was once protected by a portcullis and murder hole. Each tower rises three storeys, with circular chambers on the upper floors that would have been accessed via the curtain wall walks. The ground floor chambers feature tall splayed loops with dressed sandstone jambs, whilst the upper levels contain a mixture of original medieval features and later modifications, including some modern doorways and windows added during various repair campaigns in 1327, 1585, 1608, 1618, and 1624.

Today, much of the castle shows the scars of its long history; the south-east angle tower is completely missing, replaced in the early seventeenth century by a rectangular bastion that now survives only in fragments, whilst the east curtain wall has vanished entirely. The surviving towers and walls have been lowered and topped with modern parapets, and many of the interior spaces show evidence of repeated alterations. Despite these changes, the castle remains an imposing reminder of Anglo-Norman power in medieval Ireland, its strategic position controlling both the river crossing and the approaches to the walled city. The mixture of red and yellow sandstone used for door and window jambs provides subtle colour contrast to the grey limestone walls, whilst details like the pointed arches, barrel vaults, and spiral staircases speak to the sophisticated military architecture of the early thirteenth century.