Martello towers, thick-walled cylindrical fortifications constructed across Ireland and Britain from around 1804 onward as a defence against Napoleonic invasion, were typically sited for maximum visibility. At Tankardstown, however, the builders chose a spot that already carried a history of its own, raising their tower directly on top of an earlier earthen mound whose origins were already obscure at the time of construction.

The evidence for this earlier feature comes from two sources. A map drawn by Duncan for a proposed post road between Dublin and Derry, produced in the early nineteenth century and now held in the National Archive, clearly shows a mound at the location before the tower was built. Around the same time, a manuscript dated to 1804 refers to the Martello tower as having been constructed on part of an old Danish fort, a phrase that reflects the tendency of the period to attribute any unexplained earthwork to Viking or Norse activity, whether or not that attribution was accurate. The map itself was brought to researchers' attention by Colin Byrne of Briarleas, County Meath, and the record was compiled by Geraldine Stout and updated by Christine Baker.

The mound beneath the tower is not visible as a distinct feature today, having been absorbed into the fabric of the later construction, which means that what a visitor is really looking at is a layered site rather than an exposed ancient monument. The tower carries the national monument reference DU002-004----. Anyone with a particular interest in early earthworks or the archaeology of mound sites in the Dublin area would want to approach this one with an awareness that the physical record has been substantially altered, and that the pre-existing feature survives more in the documentary record than in the landscape itself.