The fort at Glasleck, on the shores of Milltown Lough in County Cavan, applies the same logic to quieter terrain: a long, low tongue of land pushing out into an inland lake, its neck cut off by earthworks to create an enclosure that is neither obviously dramatic nor easy to dismiss. The classification, inland promontory fort, sounds almost like a contradiction, but it describes a genuine and relatively rare type, where a lakeshore spit substitutes for a sea cliff.

The enclosure itself is roughly subrectangular, measuring approximately 42 metres on its longer axis, and it was recorded on the Ordnance Survey maps of both 1836 and 1876, suggesting it was a visible and recognisable feature well into the nineteenth century. What separates the interior from the mainland is a scarp, essentially a steep earthen slope, accompanied by a wide, shallow external fosse, a ditch, running about four metres across. Today this barrier is only legible along its southern and south-western arc; the rest has eroded or been disturbed. In 1942, a surveyor named Davies, working for the Irish Tourist Association, noted what he described as a rather sunken causeway on the southern side, and he interpreted this as the probable site of the original entrance. Whether this slight depression was always a formal causeway or simply a point where the earthwork had settled and softened over centuries, it is the closest thing to a threshold that the site still offers. Roughly thirty metres to the south-east lies a separate large rectangular enclosure, a companion feature whose relationship to the fort remains unexplained.