The tower is long gone, its site absorbed into one of the most thoroughly redeveloped stretches of Dublin city, yet the documentary record it left behind is unusually detailed, tracing the building's life across three centuries of leases, measurements, municipal arguments, and changing ownership. On John Speed's 1610 map of Dublin it appears as a square structure set into the line of the city walls, and on the Ordnance Survey map of 1863 it is recorded, with a certain elegiac precision, simply as 'Fyan's Cas. (Site of)'.

A mural tower, to use the technical term, is a tower built directly into or projecting from a defensive wall, and this one was first noted in 1305 when the Common Council of Dublin described it as the small tower opposite St. Olave's church on Fishamble Street. In 1456 a merchant named John Marcus was granted the tower for thirty years at sixpence annually, on condition he keep both it and the fish slip in repair. By 1557 it had passed, in fee ferme, to Richard Fyan, an alderman whose family had been prominent in Dublin civic life for generations; his father John had served as Mayor in 1472 and 1479, and Richard himself held the mayoralty in 1549 and 1564. A 1585 survey recorded the tower in precise terms: square, four storeys high, measuring roughly 11.6 metres one way and 6 metres another, with walls 1.2 metres thick, two arrow loops in the lower storey, and windows in the upper rooms. In the seventeenth century it passed to George Proudfoot, a merchant with connections to the first Earl of Santry, and under the name Proudfoot's Castle it was used occasionally as a state prison during the reign of Charles II.

Nothing of the structure survives above ground. The site lies in an area of Dublin that was extensively excavated in the 1970s and early 1980s before construction of the Dublin Corporation civic offices, work that uncovered significant Viking-age remains but also erased much of what remained of the medieval quayside. Visitors interested in the broader context of Dublin's city walls can trace fragments of the medieval circuit elsewhere in the city, particularly near Cook Street and along the southern approaches to the old town. Speed's 1610 map, which clearly shows the castle marked as number seven along the Wood Quay frontage, is accessible online through Cambridge University Library's digital collections and gives a reasonable sense of how the tower sat within the wall line before the riverbank was built over and the Liffey narrowed to its present course.