What makes it worth a second look is its construction: the principal wall, still standing to about 1.2 metres in height, is made not of the usual coursed rubble but of large boulders and flagstones set vertically on edge, giving it an almost deliberate, architectural quality that sits oddly with its supposed workaday function.

The enclosure was first noted by a researcher named Holt in 1911 to 1912, who recorded the curved wall of upright flagstones and mentioned the local belief that it served as a cattle pen. When the site was inspected again in May 2008, the full outline became clearer: an oval measuring roughly 32 metres north to south and 24 metres east to west. The surviving wall, running from the west-southwest around to the north, accounts for about 19 metres of that circuit. Beyond that arc, the boundary drops to a low, sod-covered wall on the northern to southeastern side, and on the remaining stretch it has been levelled entirely. A small subsidiary enclosure sits against the inner face of the wall at the north-northwest, and the ground inside slopes gently toward the northeast. To the south, a field bank ties into a wider field system that once organised this stretch of commonage, suggesting the pen was part of a broader agricultural landscape rather than a lone, isolated structure.

Tawin is a small tidal island, accessible by a causeway, and the enclosure sits within an area of commonage that still has a quiet, open character. The upright-boulder technique is the detail most worth seeking out at close range; it is unusual enough that the question of whether this is simply an eighteenth-century cattle pen, or something older that was later reused or misidentified, has never been fully settled.