Behind the hole, secured by a metal bar, sits what appears to be a cannon ball. The painted label beside it reads 'Castle Cannon Ball', and an older marble sign nearby identifies the site as the ruin of O'Dowd Castle, Lochtar Rath, described as a seat of the Kings of Tirerach. The problem is that signal towers of this type were never equipped with cannon, and the origin of the object remains unexplained. It sits alongside a 2006 memorial plaque to those who worked at the tower and those who perished at sea, giving the south wall an accretion of local memory that has little to do with the tower's original military purpose.

The Rathlee tower was built around 1804 to 1806 by the British Board of Ordnance, as part of a chain of over eighty signal stations running clockwise from Dublin Bay all the way to Malin Head in County Donegal. The system was designed to relay warnings of an approaching French invasion fleet using naval signal posts, with each tower in visual contact with the next along the coast. Rathlee sits roughly 820 metres from the shore on level ground, and its neighbours in the chain, the ruinous tower at Lenadoon Point to the north-north-west and the better-preserved one at Carrowmably to the north-east, are no longer visible from the site, blocked by buildings and forestry respectively. The threat of French invasion eased by the mid-1810s and the network was abandoned. The tower itself is a compact square-plan rubble stone structure, its interior originally divided into a semi-basement, a ground floor with a mezzanine, a first floor, and a low attic. Fireplaces with flanking alcoves survive on the south wall at two levels; the render in the ground floor alcoves still carries the impressions of shelves, and a curved stone seat is built into the lower part of one of them. The entry on the north wall sits at first-floor level and would have been reached by a retractable ladder, with a narrow machicolation, a slot above a defended doorway from which objects or liquids could be dropped on intruders, projecting above it on corbels.

The site has accumulated further layers of history. A small lime kiln, possibly used during the tower's construction, survives in a bank fifteen metres to the north, and a well-preserved Second World War lookout post, designated L.O.P. 65, stands only five metres away, a reminder that the same stretch of Atlantic coastline warranted watching again a century and a half later. An undated kitchen midden lies close to the north-east, and the possible site of a souterrain, an underground stone-lined passage associated with early medieval settlement, sits some thirty-eight metres to the north-west, suggesting the ground around this tower has been in use, in one way or another, for a very long time.