Howth Martello Tower, Abbey Street, Howth, Co. Dublin

What catches the eye first is the entrance: raised well above ground level and reached now by a modern metal staircase, it sits beneath machicolation, a projecting stone gallery pierced with openings through which defenders could once drop stones or worse onto anyone unwise enough to approach the base. The whole arrangement, door in the air, ladder or steps as the only way up, was never an accident of design. It was the point.

The tower dates to around 1805, part of a chain of coastal defences thrown up along the Irish and British coastline in response to the very real fear of a Napoleonic invasion. They take their name, oddly enough, from a tower at Mortella Point in Corsica that gave British forces a difficult time during the siege of 1794; the design was copied and adapted with enthusiasm afterwards, and versions sprang up from Sussex to Sydney Harbour. This one follows the familiar formula: a circular plan, walls rendered over rubble or masonry construction, and a granite string course marking a horizontal band before the parapet, itself finished with granite coping. Above the entrance, five granite console brackets, corbels shaped like inverted brackets, carry an ashlar granite projection that houses the machicolation. The roof is concealed entirely behind the parapet, giving the tower its clean, uninterrupted profile from a distance.

At ground level a second doorway breaks the pattern, fitted with a Gibbsian surround, granite blocks alternating with recessed panels in the manner popularised by the architect James Gibbs, and a studded metal door set with no particular symmetry in its placement. The window openings throughout are small and square headed, finished with rendered hood mouldings that would once have helped shed rainwater away from the openings below. Functional rather than decorative, these small details are what mark the tower out as a piece of military engineering first and an architectural statement only by accident, built by men thinking less about appearance than about the arc of a musket ball or the trajectory of a cannon shot from the sea.