It measures roughly 152 metres east to west and 123 metres north to south, and from ground level it reads as little more than a shallow, curving ditch, flat-bottomed and only a few centimetres deep in places. There is no signage, no obvious function, and a solitary trough sitting in the middle of the enclosed area that raises more questions than it answers. What looks, at first glance, like a vague depression in the ground turns out to be a carefully shaped landscape feature whose origins reach back to the formal gardens of the eighteenth century.

The enclosure appears on the first edition Ordnance Survey six-inch map of 1837, already showing its oval form, and aerial photography has confirmed it as a ditched feature. The more revealing document, however, is John Rocque's map of County Dublin, published in 1760, which shows the site as part of the extensive gardens belonging to the Chief Secretary's house. The Chief Secretary was the senior British government official responsible for administering Irish affairs, and the house attached to that office sat within what is now the park. A causeway in the north-east of the enclosure opens onto what appears to be an avenue-like feature leading back towards that house, suggesting this oval was once a deliberate and integrated element of a designed landscape, the kind of formal garden geometry that was fashionable among the Anglo-Irish establishment during the 1700s. The site was compiled and assessed by archaeologist Geraldine Stout, who noted that its size, shape, and shallow fosse compare well with other landscaped areas within Phoenix Park.

The enclosure sits in the Castleknock area of the park, in flat terrain to the north-west of the papal cross, which itself serves as a useful landmark when orientating yourself. Later trackways cut across the site, so the oval outline is not immediately obvious from the ground; it takes a moment of patient looking, perhaps walking the perimeter, to register the slight but consistent rise and fall of the fosse, which runs between 3.2 and 4 metres wide. There is no formal access point or managed path specifically to this feature, but the park is open to the public throughout the year, and the ground in this area is generally level and easy to cross on foot.