Some archaeological sites announce themselves with standing stones or grassy mounds. Others are legible only from the air, as faint discolourations in a crop field that betray the buried outlines of something far older. The enclosure at Castlebrown in County Kildare belongs firmly to the second category. It exists, for now, as a cropmark, the kind of ghostly imprint that appears when differential soil moisture causes crops above buried features to ripen at a slightly different rate to those around them, briefly rendering what lies beneath visible to a camera at altitude.

What the aerial photograph taken in June 2018 reveals is a partial oval enclosure of roughly 39 metres east to west and 28 metres north to south, with two concentric ditches or banks, making it what archaeologists call a bivallate enclosure. A smaller annexe, approximately 18 by 9 metres, projects from the north-east side. Bivallate enclosures of this general type are associated in Ireland with the early medieval period, though without excavation it is impossible to assign a firm date to this particular example. One detail that does carry a date is the field boundary that cuts across the north-west quadrant of the enclosure. That boundary appears on the first edition Ordnance Survey six-inch map of 1838, which means the enclosure was already partially obscured or bisected by agricultural reorganisation at least that far back, and probably long before anyone thought to map it.