The one at Williamstown, numbered Tower No. 15 in the coastal sequence, quietly breaks that rule. At approximately 44 feet in diameter it is notably larger than its neighbours, and its parapet, rather than sitting directly on the main body of the tower, is carried on two courses of quadrant-shaped corbels, projecting stone brackets that give the upper section a distinctly unusual profile. A machicolation, the kind of projecting gallery that would have allowed defenders to drop objects on anyone trying to force the door, may once have featured here, but no trace of it remains. What does remain is a blocked-up tall narrow opening on the west side, almost certainly the original entrance, later sealed and replaced by the current doorway in the south face.

The tower is one of fourteen actually built in a coastal chain numbered from 1 to 16 south of Dublin, a numbering scheme that sounds contradictory until you learn that two positions in the sequence had artillery batteries but no towers at all. Construction across the whole network began in 1804 under Colonel Benjamin Fisher of the Royal Engineers, and by December 1805 every tower had been armed and was considered complete. The pace was driven by the perceived threat of Napoleonic invasion, and the towers were designed to provide mutual support along the shoreline, each one within signalling distance of the next. Taylor's survey map of the Dublin environs, produced in 1816, records the tower's position clearly, and a drawing attributed to the artist Samuel Frederick Brocas captures the broader coastline at Blackrock in the same period, with two Martello towers visible against the sea.

The tower stands on an east-facing slope with views northward along the coast towards Blackrock, now within a park that borders the railway line. It is free-standing and built of dressed granite. Because the site sits close to the suburban rail corridor, the most practical approach is on foot from the Blackrock or Seapoint direction, following the coastal path. The double-corbel parapet is the detail most worth pausing over from the exterior, the two projecting courses giving the roofline a slightly tiered appearance that sets it apart from the plainer profiles of the other towers in the chain.