The enclosure at Castle Ellen appears designed to look like a moated site, the kind of water-filled defensive ditch that surrounded fortified dwellings across medieval Ireland, but the cartographic evidence suggests it was built in the Victorian era, long after such features had any defensive purpose. The whole thing, it seems, was an exercise in deliberate antiquity.

A moated site, for context, is an enclosure defined by a wide ditch, sometimes water-filled, dug around a residential or agricultural complex, and they are a recognised feature of the medieval Irish landscape. What makes Castle Ellen unusual is that someone appears to have built one from scratch, not to defend anything, but possibly as a designed landscape feature, a folly in the literal sense: an ornamental construction built for aesthetic effect rather than function. Cody, writing in 1989, examined the Ordnance Survey maps and found that the enclosure appears on the 25-inch OS plan of 1892 and again on the 1927 to 1928 revision, but is entirely absent from the original 6-inch OS map and its Fair Plan, placing its construction somewhere between 1839 and 1892. The enclosure is roughly rectangular, measuring approximately 27 metres northeast to southwest and 21 metres northwest to southeast, defined by two low earthen banks with an intervening fosse, the term used for the ditch between such banks. The outer face of the inner bank is vertical and revetted, meaning faced, with drystone walling that remains well built and almost completely intact. Causewayed gaps, essentially raised crossing points, break the fosse on both the northeast and southwest sides, though one of these appears to be a later addition. Trees have grown along the southern and western sides of the inner bank, adding to the impression of enclosure.

At the western end of the interior, largely indifferent to whatever the Victorians intended around it, stand the genuine remains of a medieval tower house and a section of its associated bawn wall. A bawn was a defensive enclosure wall surrounding a tower house, used to protect livestock and provide a defended yard. These earlier ruins predate the folly by centuries, and their presence inside the constructed enclosure gives the whole composition a slightly theatrical quality, as though the landscape's designers were framing something real with something invented.