Archaeological excavations in 1996 revealed that this site has been continuously occupied since prehistoric times, beginning with a rectangular building whose inhabitants left behind stone axe heads and flint tools. The early medieval period saw the construction of small round houses, each about five metres across, built with post and wattle construction. One of these dwellings even featured a curved wooden stockade, and the residents began cultivating the land with regularly spaced furrows that eventually overwhelmed the settlement.

When the Anglo-Normans arrived around 1175, they transformed the site dramatically. Maurice Fitzgerald, who received the Manor of Maynooth from Strongbow, erected the stone castle in the late 1180s. The original keep measured roughly 23 metres by 19 metres, with walls over two metres thick, and featured an entrance positioned defensively at 6.3 metres above ground level. The castle became the principal residence of the Kildare Geraldines at the beginning of the 14th century, and in 1426, the sixth Earl, John, undertook significant alterations including the insertion of two barrel vaults supported by a new spine wall. The castle's importance continued into the 16th century, when the ninth Earl sought permission to establish a college in 1518, though the structure suffered severe damage during the Silken Thomas Rebellion and was eventually dismantled during the 1641 rebellion.

Today's visitors can still see the rectangular keep, gatehouse, southeast tower, and fragments of the great hall and gallery. The excavations uncovered fascinating artefacts from various periods, including Ham Green pottery, iron spurs, and scabbard chapes from the Norman period, alongside several intact medieval jugs and wooden objects preserved in the castle well. Even the kitchen refuse tells a story; animal bones reveal that the castle's residents dined on fallow deer from their own deer park, as well as young seal and other unusual fare. After its military days ended, the keep found new life in the 18th and 19th centuries when half the ground floor served as a coal store whilst the exterior wall became a handball alley, proving that even ancient fortresses can adapt to modern recreational needs.