This earthwork fortress served as the chief manor for le Petit’s extensive lands, which included the barony of Magheradernon, and commanded a strategic position on high ground overlooking a crucial fording point across the River Brosna just 90 metres to the southeast. The motte’s influence on the town’s layout can still be traced today; the distinctive curve in Ballinderry Road near its junction with Mount Street likely follows the circular base of the long-demolished mound, a ghostly reminder of medieval power frozen in modern tarmac.

The castle’s long history saw it evolve from earthwork to stone fortification, with a later stone castle known as Mullingar Castle constructed atop the motte’s summit. During the Williamite Wars of 1691, the ancient earthwork found new purpose as a cannon emplacement, its elevated position proving as valuable for artillery as it had been for medieval defence. Richard’s map from that year intriguingly labels both the motte and what appears to be its detached bailey to the west as “Danish forts”, though they were decidedly Norman in origin. The map reveals fascinating details: steps built into the north face of the circular mound provided access to the summit, where the remains of a polygonal structure, possibly the later stone castle, still stood.

This remarkable survival of nearly 650 years came to an abrupt end in 1828 when the entire complex was demolished to make way for the County Gaol, itself later replaced by the current council offices. The castle had been part of a medieval landscape that included a watermill just 50 metres away on the River Brosna, a parish church 130 metres to the northeast, and the main street of Mullingar town 190 metres north. By 1653, it was described as “one great castle with a garden thereunto belonging”, still garrisoned and central to the town’s defence. Today, nothing visible remains of le Petit’s mighty fortress, though its presence shaped Mullingar’s development for centuries and its footprint lies buried beneath the administrative heart of modern Westmeath.