Historical records from Bradley et al. (1986) and de Burgh (1895) describe the building as having a barrel vault over its ground floor, a characteristic feature of defensive tower houses common in medieval Ireland. Though the castle met its end during demolition in 1973, one of its walls remained attached to a modernised two storey building for some years after, serving as a tangible reminder of the town’s medieval past.

Archaeological investigations in 2002 and 2004 have helped piece together the castle’s footprint and its relationship to Naas’s medieval town defences. During assessment work for a development at Poplar Square, archaeologists discovered remnants of the castle despite significant modern disturbance from previous pipe and cable laying. The excavations revealed sections of rubble walls covered with mortar, organic soil deposits with flat stone fragments that may represent old ground surfaces, and areas of cobbling set in dark brown, sandy clay flecked with charcoal. Most significantly, the 2004 excavation exposed a portion of the castle’s eastern wall; a substantial structure measuring 1.55 metres thick and built from large green subrectangular blocks, though its centre had been truncated by modern service pipes.

The castle’s location at the southwest corner of the development site places it near the presumed line of Naas’s medieval town defences, which ran northwest to southeast across the eastern portion of the area. A large ditch exposed during excavations has been interpreted as part of these defensive works, suggesting that Eustace Castle may have played a role in the town’s fortifications. While the northern wall was likely destroyed during the construction of Lawlor’s Hotel in the twentieth century and the western wall may lie beneath the current footpath, archaeologists believe the southern wall could still survive within an adjoining property, awaiting future discovery.