Lynch's Castle, 40, Shop Street, Abbeygate Street Upper, Galway, Co. Galway

It is an odd, almost private joke set into the face of what was once the grandest merchant house in Galway, and it survives on a building that now does business as a bank, its ground floor given over to teller windows rather than the comings and goings of a sixteenth-century trading family.

The tower house was built around 1500, at a time when Galway was ruled by a tight circle of merchant families, of whom the Lynches were the most prominent. What stands today is a four-storey corner building, though it was originally five storeys; the floor levels were altered around 1820, when the tidy, evenly spaced Georgian sash windows now visible on the upper floors were inserted. Traces of the earlier arrangement remain if you know where to look: arrow-loops at the corner, and blocked window openings sitting between the current floor levels, mark where walls once met different floors. The carving that survives, however, is what sets the building apart. Round panels on the front and side elevations carry the Lynch coat of arms and, on the Abbeygate Street side, the Fitzgerald arms, each accompanied by a carved lion with an oddly human face and lines of black-letter script beneath. The Henry VII panel is more elaborate still, framed and finialled, with a Latin inscription in Gothic lettering wishing long life to the King of England, France and Lord of Ireland, a nod to Galway's formal loyalty to the crown even as the city ran itself as something close to an independent merchant republic. This density of decorative stonework on a domestic building, rather than a church or civic structure, is unusual in an Irish context, and no comparable example survives in as complete a state. Inside, an entrance hall retains a chimney-piece whose keystone carries the IHS monogram, a set of initials, and the date 1629, a later addition marking the building's continued use long after its defensive purpose had faded.

The ground floor's arcaded doorways and round-headed windows, with their carved capitals and hood-mouldings ending in small human faces, were inserted in 1933, a twentieth-century intervention that gave the building its current banking-hall entrance while leaving the medieval stonework above largely undisturbed. Machicolations, meaning projecting parapets with openings through which defenders could drop objects on anyone below, survive on the side elevation on carved corbels, alongside a garderobe chute, evidence of the tower house's original function as a fortified urban dwelling rather than simply a shopfront. A scar in the north-west gable hints at a building that once stood alongside it and has since disappeared, one more layer in a structure that has been added to, cut into, and reworked for five centuries while still keeping its carved faces turned outward onto the street.