The fosse, a wide external ditch that once defined the perimeter, is still readable from the south-east and south-west, and the interior, which tilts gently from west to east, still carries faint traces of old cultivation ridges. But the most intriguing feature lies in the western half of the enclosure: the footings of a stone structure with an internal division, and to its north, a square annexe with a gap in its eastern side. The question of what exactly these remains represent has puzzled antiquarians and surveyors for well over a century.

The stone building within the fort may be the remains of Loughanstown Castle. The castle appears on the Down Survey map of Portnashangan parish, drawn between 1655 and 1659, but it disappears from the record entirely after that, absent from every edition of the Ordnance Survey six-inch maps. By the nineteenth century, even the walls had gone. John O'Donovan, surveying Westmeath for the Ordnance Survey, noted only the site of a castle that had belonged to a Dr Delamare, adding that a green earthen mound occupied the spot and that the small lake giving the townland its name lay in the next field. A separate Ordnance Survey Name Book entry, recorded as number 47, described the site as lying immediately to the left of the road from Loughanstown to Mullingar, very close to what it calls the lockan, the Irish word for a small lake. The castle ruins still appeared on William Larkin's 1808 map of County Westmeath, which suggests some visible fabric survived into the early nineteenth century. The difficulty is that the officially recorded location for the castle sits in a field to the north-north-west, where there is no surface evidence at all to support the identification. The earthwork to the east of the lake, with its stone footings inside, is now considered the more plausible candidate. Whether the ringfort, an enclosed circular settlement of the kind built across Ireland from the early medieval period onwards, was already ancient when the castle was constructed and simply pressed into service as a bawn, a walled or banked courtyard used for enclosing livestock and providing defence, or whether both were built together as a single planned complex, remains an open question.