Knockaulin, a round-topped hill rising to about 183 metres above the Kildare plain and overlooking the Curragh to the north-west, carries one of the least-visited royal sites in Ireland. Known in early Irish literature as Dún Ailinne, it was the ceremonial seat of the kings of Leinster, described in medieval texts as a place of assemblies, royal roads, and a grianan, meaning a palace or sun-chamber. The name itself may derive from an 'ail', an Irish word for stone, referring to some feature once visible on the hill. What makes the place quietly unsettling is the gap between its literary reputation and its present appearance: the great enclosing bank and fosse, a rampart still standing around five metres high and enclosing roughly thirteen hectares in a broad oval, is easily walked around, but virtually nothing above ground survives to suggest what once stood inside.

Seasonal excavations carried out between 1968 and 1975 revealed a sequence of activity reaching back to the Neolithic period, including a small enclosure, a possible burial, and traces of habitation. The most substantial findings, however, were Iron Age, and they were extraordinary. Three successive phases of large timber construction were uncovered on the hilltop, each partly cutting through the one before. The excavator named them the White, Rose, and Mauve phases. The White Phase was a circular palisade trench about 22 metres in diameter, dismantled and replaced by the far more elaborate Rose Phase: three concentric timber circles enclosing an inner area of 28.5 metres in diameter, with posts graded by size from inner to outer ring, a north-east entrance flanked by diverging fences and a post-lined avenue, and a smaller conjoined enclosure to the south. This too was dismantled and replaced by the Mauve Phase, two concentric circles with an external diameter of about 42 metres, a central ring of large free-standing posts, and an innermost circular trench with no obvious entrance. Each structure was taken down deliberately. Deposits left after the final dismantling included layers of animal bone, predominantly cattle and pig, along with burnt stone, charcoal, and ash, suggesting episodes of communal feasting. Finds including an iron sword, an iron spear-head, iron needles, bronze fibulae, and glass beads point to ritual use of the site between roughly 390 BC and 320 AD. The outer rampart, a fosse with a bank set on its inner side rather than outer, which is characteristic of a class of monument sometimes called a hillfort or royal enclosure, was probably constructed after 700 BC according to radiocarbon dating of material beneath it. When John O'Donovan visited in 1837 while compiling the Ordnance Survey Letters for County Kildare, he recorded a rath on the summit and a holy well, St John's Well, in the north-west sector, both of which appeared on the first edition six-inch Ordnance Survey map of 1838.