A large defensive ditch, uncovered during excavation in 1990, appears to have marked the boundary of the medieval town, and alongside it lay a contemporary property boundary ditch, suggesting that the edge of the borough was also the edge of individual landholdings. Nothing of this is visible today, yet the place retains traces of its fortified past in unexpected ways, not least in a street name.

The story of Tallaght's defences begins with a royal grant, issued in 1310 or 1311, authorising the construction of a defensive wall around the borough. In 1311, a murgage grant, that is, a formal permission to collect a toll on goods brought to market in order to fund the building and upkeep of town walls, gave the bailiffs and good men of Tallaght the right to levy this tax for three years. How much was actually built remains unclear, but the infrastructure of a defended medieval town was evidently taking shape. A gate-tower is thought to have stood at what is now called Watergate Bridge, where the Old Bawn Road meets a natural boundary line that once separated the burgage plots, the long narrow strips of land allocated to townspeople under medieval borough law. The gatehouse itself is recorded in the archaeological inventory. Woven into all of this is a piece of local tradition known as Talbot's Leap, in which one Talbot of Belgard, fleeing Cromwellian forces, arrived at a drawbridge to find it lowered against him and was forced to jump the fosse, the defensive ditch it spanned, in order to escape. The story is unverifiable, but it points to a memory of a functioning fortified crossing at or near the Watergate site.

There are no visible surface remains. The ditches, the wall, the gate-tower, the drawbridge: all of it lies below or has been removed entirely. What a visitor can do is stand at the junction of the Old Bawn Road and consider the placename Watergate Bridge, which quietly preserves the memory of a medieval entrance to the town. The name is the monument here, and knowing what it once marked changes how ordinary a stretch of road it seems.