The other three points were demolished when the ESB built and fenced off their electricity station along the northern section of the site, leaving the geometry of the original design readable only in part, and only to someone who already knows what to look for. The 1840 to 1841 Ordnance Survey map recorded it clearly as a small star fort, labelling the site 'Massy's Hill'; later editions of the same map show the outline with less and less precision, as though the cartographers themselves were losing confidence in what remained.

A star fort, to give the form its context, is a type of early modern fortification designed so that every section of its outer wall could be covered by fire from adjacent projecting points, eliminating the blind spots that plagued older circular or square enclosures. This one measured roughly 250 feet across, with between 125 and 150 feet between its salient angles. Two stone banks and a fosse, the fosse being the ditch between them, still define the southern and eastern sections. The inner bank rises up to four metres above the fosse at its highest surviving point; the outer bank reaches about a metre on its external face. A break in the bank to the south-east may be an original entrance, though the extent of later disturbance makes that uncertain. The plan form points to construction somewhere between the early and mid-seventeenth century, and scholars have noted a resemblance to the tenaille-style earthworks constructed at Dunboy Castle in County Cork by Lord Broghill during the 1641 to 1660 period. That comparison has led to suggestions of either Confederate or Cromwellian origins, though nothing has been firmly established either way.