Hatch's Castle, 48, Market Street, Ardee, Co. Louth

It stands slightly forward of the buildings flanking it to north and south, as though the street had grown up around a structure that refused to align itself with later fashions. Look closely and the working life of the building shows through: a pointed arch window lights the top floor, while the storeys below make do with plainer square-headed openings, each fitted with heavy stone hood mouldings to throw off rainwater. The whole effect is of a small fortified house that has simply kept going, floor by floor, century by century, while everything around it was rebuilt.

The tower dates to around 1600, placing it at the tail end of Ireland's tower house building tradition, when minor lords and merchants across the country still favoured a defensible stone residence over an open house. Its rectangular plan carries a semi-circular bow on the east side and a rounded tower at the south-east corner, both softening what would otherwise be a hard-edged block, and rounded corners on the main walls do similar work, a common device for deflecting both weather and artillery from vulnerable angles. The walling is random limestone rubble, unpolished and workmanlike, and the roof itself is concealed entirely behind the parapet, so that from the street the building reads as solid stone from base to battlement. A rubble stone chimneystack, corbelled outward and later patched with red brick, still rises above the roofline, and the depressed arch doorway, with its chamfered surround and limestone hood moulding, gives onto a timber door of seven pointed-arch panels beneath a plain glazed overlight, a piece of joinery that has clearly been renewed more than once without disturbing the older stone frame it sits in. At some point the tower was drawn into domestic use as a private residence, a conversion that has left its defensive skeleton intact while quietly filling it with the comforts a sixteenth-century garrison would not have known.

What holds the composition together at street level is easy to miss if you're not looking for it: a run of wrought-iron boundary railings on a moulded limestone plinth, its uprights alternating between straight bars and a sinuous, snaking pattern, an unexpectedly decorative flourish for a building otherwise built for endurance rather than display. Because the tower is still lived in and sits hard against its neighbours in an ordinary streetscape, it rewards a slow look from the pavement rather than any formal visit, the kind of building you notice only once, and then wonder how you ever walked past it before.