No walls, no tower, no fosse. Just postholes, boundary ditches, plough furrows, kilns, and a single hearth, all cut into the natural mottled sand and clay of a north-facing slope at the edge of a field. The absence of defensive features is, in its own way, the most telling detail.

Recorded by Breen in 2012, the structure known as Structure A was uncovered in the western perimeter of the excavated Southfield at Carrickmines Castle (DU026-005001-), detectable across Trenches T2, T5, and T6. It represents the earliest phase of medieval activity on the site, preceding both a horizontal mill and the castle itself. The feature set is extensive for something so slight in the ground: two slot-trenches or gullies, four boundary ditches, five narrower slots interpreted as possible internal divisions, three postholes, a hearth, four pits, a quarry pit, and three circular kilns thought to indicate metal- or glass-working. The plough furrows to the west of the main gully suggest a farmhouse or agricultural outbuilding rather than anything grander. Two distinct sub-phases of activity were identified, marked by the replacement of one gully with another of a different orientation, and adjustments to the boundary ditching. The whole sequence was eventually cut through by a large linear ditch, F82, which itself was later truncated by the headrace of the horizontal mill that replaced this phase of settlement. Leinster Cooking Ware and Dublin Type Ware pottery recovered from the fills help place the occupation within the medieval period, though a precise date range is not specified in the excavation record.

Carrickmines Castle is better known to many Dubliners as the site of a prolonged and contentious planning dispute in the early 2000s, when road construction threatened the archaeological complex. The excavated remains of Structure A are not visible above ground, and the site is not presented as a visitor destination in the conventional sense. Those with an interest in the archaeology can consult the published record through Breen's 2012 report. The surrounding area retains some of its fieldscape character, and knowing that somewhere underfoot a medieval farming settlement once organised itself around ditches and kilns, long before any castle gave the place its name, does change how the landscape reads.