According to a lease document from 1608, Eustace was obligated to construct a proper fortified castle at Gurtinavackan, complete with specific architectural requirements. The lease stipulated that he must ‘build the said castle of Gurtinavackan substantially with lime and stone, a story height above the vault, with a battlement, and a slate roof’. Beyond the castle itself, Eustace was also required to fortify the surrounding settlement by building a stone wall around the town and installing a sturdy gate made from oak timber and boards.

The site of this once substantial castle is now marked by a circular mound, which was recorded on the 1907 Ordnance Survey six-inch map. This earthwork represents all that remains visible above ground of what was once a defensive stronghold built to secure English control in the area during the plantation period. The mound itself likely contains the foundations and lower structures of Eustace’s castle, buried beneath centuries of accumulated earth and vegetation.

Archaeological evidence suggests the site had even earlier defensive features. In an adjacent field to the south, aerial photography has revealed cropmarks indicating a rectangular enclosure, most likely a medieval moated site (recorded as KD040-030). This earlier fortification would have predated Eustace’s castle, suggesting that Gurtinavackan held strategic importance for several centuries. The presence of both structures indicates this location was repeatedly chosen for defensive settlements, making it a significant, if now largely forgotten, piece of Kildare’s military heritage.