This barrow probably looks quite unassuming. But it does get a mention in volume 2 of Camden’s Britannia. He says:
The Wye crosses the west angle of the County; and having its rapid course somewhat abated by the rocks it meets with, and its chanel discontinu’d, it suddenly falls headlong over a steep precipice. Whence the place is called Rhaiadr Gwy, that is, the Cataract or fall of the River Wye. [...] About two furlongs below [the Castle] I observed a large Tumulus or Barrow, call’d from a Chapel adjoyning, Tommen Iban St. Fred: and on the other side, at a farther distance, there are two more, much less than the former, called Krigeu Kevn Keido, vix. the Barrows of Kevn Keido, a place so call’d; where it is suppos’d, there stood heretofore a church, in regard a piece of ground adjoining is call’d Klyttieu’r Eglwys.
This is from p699 of the 1753 version, but he originally published it in 1607. Cefn = a ridge.