This fortification served as the chief manor of his extensive lands, which included the barony of Magheradernon, and strategically controlled the river crossing just 90 metres to the southeast. The castle’s prominent position also placed it near other significant medieval structures; a mill site operated 50 metres away, whilst a parish church stood 130 metres to the northeast, with the main street of medieval Mullingar developing just 190 metres to the north.

Over time, a stone castle known as Mullingar Castle or Petit’s Castle was built atop the original motte, transforming the earthwork fortification into a more permanent stronghold. Historical records from 1582 list it among the forts of Leinster, and by 1622, an inquisition into the estate of the deceased Redmond Petitt referred to it as ‘Pettitt his castle’. A 1667 land grant confirmation intriguingly described the site as ‘One Castle called ye King’s Castle consisting of 2 Castles & one Halle’, suggesting a complex of structures had developed on the site. Richards’ map from 1691 depicts the motte as a circular mound with steps built into its north face, crowned by what appears to be a polygonal structure, likely the remains of the stone castle.

The castle’s long history came to an abrupt end in 1828 when the entire earthwork was demolished to make way for a new county gaol. Today, Westmeath County Council Offices occupy the site, with the original location of the motte castle believed to have been closer to the junction of Mount Street and Bleach Yard, around the entrance area of the present council buildings. Interestingly, the Ordnance Survey 25-inch map incorrectly marked Petit castle’s location, a cartographic error that has since been corrected by historical research, ensuring this important piece of Mullingar’s medieval heritage isn’t lost to memory despite its physical absence from the modern landscape.