There is a circular enclosure at Castlebrown in County Kildare that no one standing on the ground has ever seen. It exists, as far as any observer is concerned, only from the air, revealed as a cropmark, the faint differential in how grass and grain grow where buried features disturb the soil beneath. What aerial photography captured here is roughly half of what was probably once a complete ring, somewhere around thirty metres across at its widest point.

The surviving evidence comes from aerial photographs taken in 1965, which show the eastern half of the enclosure as a cropmark on level pasture. The site sits at the foot of a steep slope marking the southern end of a north-to-south ridge, just east of the Gollymochy River as it runs away to the south-east. Circular enclosures of this kind are among the most common monument types in the Irish landscape; they range from prehistoric ring ditches and burial monuments to the ringforts that served as enclosed farmsteads well into the early medieval period. Without excavation, it is impossible to say which category this one belongs to, and no such investigation appears to have taken place. The western half of the ring has either been destroyed or simply left no trace readable from above.