Mill Mount Barracks, Barrack Street, Drogheda, Co. Louth

The circular structure rising in three stages above its cobbled approach looks, at first glance, like a standard piece of coastal defence architecture. But the ground beneath it is a medieval motte, the flat-topped earthen mound that once carried a timber castle, and the tower's position on top of it means two very different eras of military anxiety are stacked one on the other, centuries apart.

The tower itself dates to around 1805, part of the chain of Martello towers thrown up around Ireland's coasts during the Napoleonic Wars, when invasion by Bonaparte's forces seemed a real possibility. These squat, thick walled towers, named after a fort in Corsica that had proved stubbornly hard to capture, were built in their dozens to watch the shoreline and house small garrisons with artillery on their roofs. This one was built with ashlar limestone, cut and dressed to a smooth finish, and its upper section projects outward on stone corbels, a defensive feature meant to let defenders fire down on anyone attempting to scale the walls below. Access was originally by a removable or metal staircase to the first stage, keeping the ground floor sealed off from attackers. The tower's later history took a harder turn: it was damaged during the War of Independence in 1922, decades after its original military purpose had faded, and stood in a ruined state for the better part of a century before an extensive rebuilding in 2000 restored its conical slate roof, metal finial and stonework.

Approaching the tower today means walking a cobblestone path flanked by rubble and rendered walls, passing through square profiled ashlar gate piers with metal gates set into a limestone boundary wall, all part of the former military barracks that once surrounded it. The tower's square headed window openings, with their chamfered sills, and its timber sheeted door are the visible result of that 2000 rebuilding, faithful to the tower's original defensive form even where the stone itself is comparatively new. Little from the medieval motte survives above ground beyond the raised, elevated site the tower now occupies, but the pairing of the two, a Napoleonic watchtower on an earlier fortified mound, is what gives the place its layered, slightly improbable character.