Built in 1546 by Sir William Brabazon on the orders of Lord Deputy Bellingham, the fort rose from the ruins of an O'Conor stronghold that had been seized by the Crown just nine years earlier. The original fortress, constructed in marshland and defended by forty gunners, was strategically positioned to control this contested territory. When Uí Failge was formally shired in 1556, the growing settlement around the fort was renamed Philipstown in honour of Philip II of Spain, marking its transformation from a military outpost to a plantation town.

Today, visitors can explore the impressive remains of this nearly square bawn, measuring roughly 82 metres north to south and 83 metres east to west. The fortification's perimeter is defined by a defensive ditch and substantial stone walls, some reaching heights of 2.8 metres, with the southwest corner being particularly well preserved. Within the enclosed area, grass covered foundations trace the footprint of what appears to be an H-shaped castle that once dominated the centre of the fort. A stone causeway on the western side provides access over the fosse, though this feature likely dates from a later period than the original fort construction.

The fort's military significance is still evident in historical records; as recently as 1942, surveyors documented the vestiges of a bastion projecting from the western wall, though no trace of this defensive feature remains visible today. Two late 16th century armorial plaques that once adorned the fort have found new homes in the local library and Castlebarnagh golf club, serving as tangible links to the fortress's Tudor past. Archaeological investigations in the early 2000s, including monitoring of nearby development and test trenching at the adjacent national school, have revealed little additional material, suggesting that the visible remains represent the primary surviving elements of this once formidable stronghold.