In a field beside the River Liffey in County Kildare, there is an ancient enclosure that only reveals itself from the air. No earthwork rises above the grass, no obvious feature interrupts the landscape; instead, a semi-circular outline appears as a cropmark, the kind of ghostly impression that shows up in aerial or satellite imagery when buried features cause the vegetation above them to dry out or grow differently from the surrounding soil. It was spotted in Google Earth imagery captured on 28 June 2018, and what it shows is quietly intriguing.

The enclosure measures approximately 35 metres north to south and 27 metres east to west, with what appears to be an entrance gap on its south-south-eastern side. Its most curious characteristic, though, is its relationship with the river. The semi-circle accounts for only part of the perimeter; the eastern bank of the Liffey seems to have served as the western boundary, completing the enclosure in the same way a cliff edge or a steep riverbank might be used in cliff-edge forts, a category of monument where natural topography does the work of one side of the defensive circuit. This economy of construction was deliberate, placing the monument so that the river acted as both boundary and barrier. The site sits in grassland at Castlesize, and while the name of the townland hints at something fortified or significant in the area's past, the cropmark enclosure itself has no firm date attached to it from the available evidence.