The surviving masonry at Castlequarter sits on a low pinnacle of rock outcrop, with a turlough to the south. A turlough is a seasonal disappearing lake, characteristic of karst limestone country, which floods in winter and dries out in summer, lending the surrounding landscape an unsettled, intermittent quality. The ruin itself is compact to the point of oddity: its exterior footprint measures roughly 4.55 metres north to south and 4.65 metres east to west, while the interior shrinks to just 2.75 by 2.5 metres, leaving walls more than a metre thick.

Those proportions, along with the absence of any identifiable doorways, prompted the researcher Brew, writing in 1991, to suggest that this may never have been a windmill at all. Windmills typically present a circular or polygonal tower, designed to allow the cap and sails to rotate and catch the wind from any direction. A squat rectangular box with walls nearly a metre wide and an interior barely large enough to stand in comfortably does not fit that pattern. The more likely candidate, Brew argued, is a watchtower, a function that would explain both the elevated position on the rock outcrop and the unusually solid construction. The southern wall, standing to an internal height of just under a metre and an external height of 1.3 metres, is the best-preserved section; the remaining walls have been reduced to traces. By the time the Ordnance Survey mapped the area, whatever the structure's original purpose, it had already been in ruins long enough for local knowledge of its function to have blurred.