A mural tower, meaning a tower built into and forming part of a defensive city wall, it was known alternately as the Watch Tower and Fagan's Tower, and its purpose was specific: a sentry stood there keeping an eye on Newgate Gaol, the prison that sat close by on the city wall circuit. That combination, a private citizen's name attached to a public fortification serving a prison-watching function, says something quietly odd about how medieval Dublin managed its defences.

A survey from 1585, preserved in the Calendar of Ancient Records of Dublin, describes the tower in careful, almost fussy detail. It was round on the outside and square within, stood 32 feet high (roughly 9.8 metres), with walls 2 feet thick (around 0.6 metres) not counting the stairs. There was no vault and no loft, just a stairway rising to a top platform 10 feet square, fitted with a garret and five loops, the narrow openings in a defensive wall through which archers or watchmen could observe or fire outward. The tower sat 90 feet from Sedgrave's Tower to its east and 120 feet from the south-east tower of the New Gate to its west, with houses pressing up against the inner face of the wall along that stretch. By 1618, the structure had passed into a lease held by Alderman Thaddy Duff, at an annual rate of two shillings. The name Fagan's Tower attached to an earlier occupant, one Mr. Richard Fagan, who held it at the time of the 1585 survey.

The tower itself no longer stands above ground, but archaeology has traced something of what remains. Between October 1996 and March 1997, Tim Coughlan led an excavation across a 6-metre-wide, roughly 32-metre-long trench running east-west between Back Lane and Lamb Alley, carried out under licence 96E0300 for Zoe Developments Ltd. Seven phases of activity were identified, ranging from early postholes and cultivation layers through post-and-wattle and stave-built structures, up to later medieval and post-medieval deposits. In the western part of the trench, a section of mortared masonry of possible medieval date was uncovered, and Coughlan suggested it may represent the remains of the mural tower itself. The area today is unremarkable streetscape, but Lamb Alley runs along the line of the old wall, and the RTE archive holds footage of this section of the city wall that gives a sense of what once stood here.