10 John Dillon Street, in the south inner city of Dublin, lies the ghost of a medieval defensive tower that was filled with earth rather than air. That detail alone sets it apart from the more familiar image of a fortification: no vaulted ceiling, no upper loft, no garrison standing at arrow-loops looking out over a clear interior space. Just compacted earth inside a half-round shell of stone, four and a half metres high, with four narrow loops cut into walls roughly 1.2 metres thick. It was, by any measure, a modest structure, but it formed one small node in the much larger system of Dublin's city walls.

A 1585 survey, preserved in the Calendar of Ancient Records of Dublin, recorded the tower in meticulous detail and noted that it was then in the possession of Sir William Sarsfield, after whom it became known as Sarsfield's Tower, or simply the Round Tower. The survey measured the stretch of wall from Saint Nicholas Gate to the tower at 312 feet (roughly 95 metres), and described the broader wall as standing 16 feet (about 4.9 metres) high, with an earthen rampart behind it some 15 feet (4.5 metres) thick and a buttress rising nearly 19 feet (5.8 metres) from the bottom of the outer ditch to the wall's foundation. A further 340 feet along the same wall stood another small tower, then in the possession of a Mr. Christopher Sedgrave. The 1585 account is a mural tower survey in the truest sense: a systematic, almost bureaucratic record of a city trying to understand the condition of its own defences. A deed map from 1780 later confirmed the tower's position behind what is now No. 10 John Dillon Street.

John Dillon Street runs between Bride Street and Kevin Street Lower, close to the old line of the medieval city boundary on Dublin's south side. There is nothing to announce the tower's presence from the street; the site sits behind the building line, and no visible remains are accessible to a casual visitor. Its value lies less in what can be seen today than in the documentary record that survives around it, particularly that 1585 survey with its mixture of careful measurement and Tudor orthography. Anyone with an interest in Dublin's walled medieval circuit might pair a visit to this area with the better-preserved sections of the city walls visible nearby, and consult the Historic Environment Viewer, where the site is recorded under the reference DU018-020001-.