At Castlesize in County Kildare, a large circular enclosure roughly 90 metres across lies almost entirely invisible at ground level, yet reveals itself clearly from above as a cropmark, that is, a subtle difference in how vegetation grows over buried features, caused by ancient ditches or banks affecting soil moisture and depth. The circle is so large that a person walking through the field would have no sense of its shape or boundaries at all.

Cropmarks of this kind are typically produced by the differential growth of grass or crops over buried archaeological features, with lusher or more stunted growth tracing the lines of filled-in ditches or compacted surfaces beneath. This particular enclosure, approximately 90 metres in diameter, is among the larger examples of its type. A gap on its southern side suggests an original entrance, and a road excavation trench has cut across the site, bisecting it and offering an inadvertent cross-section through what may be a substantial prehistoric or early medieval boundary. The site came to wider attention through Google Earth imagery captured on 28 June 2018, a reminder that satellite photography has become one of the more unexpected tools of Irish landscape archaeology, surfacing features that ground surveys and casual inspection have long missed.