It was described in 1934 by Lyons as a "rectangular fortification or earthwork adjoining the River Anner", which sounds suitably martial, suitably ancient. The reality turns out to be considerably more domestic.

The earthwork lies about seventy metres west-northwest of Anner Castle, cut into a steady NW-facing hillside. It is a substantial piece of ground-shaping: a levelled platform roughly 26 metres north to south and 17 metres east to west, defined on both the upslope and downslope sides by stepped counterscarp earthworks, a term for a sloped or stepped face built to retain and define a level area against natural gradient. The lower edge of the complex is further defined by a low bank that drops steeply to the river. The hillside has been cut back about a metre on the interior at the northeast end, reinforcing the impression of deliberate, carefully maintained terracing. None of this appears on the first Ordnance Survey six-inch map of 1840, but by the second edition of 1904 the feature is clearly shown, measuring approximately 50 metres by 20 metres. The most plausible explanation is that it was created as part of the landscaped grounds of Anner Castle, and the occupants of the castle have their own tradition about what it once was: a tennis court. The geometric precision of the levelling, the retained edges, the riverside bank, all of it makes rather more sense as a late Victorian or Edwardian leisure feature than as any kind of fortification. Lyons, writing in 1934, may simply have encountered an overgrown terrace whose original function had already been forgotten.

The site now sits in mixed woodland and undergrowth, its dressed earthwork edges softened but still legible as deliberate construction. It is the kind of place that rewards a slow look, where the gap between how a thing appears and what it actually was turns out to be the most interesting thing about it.