Afon Groes Fawr valley contains many archaeological remains, including Castell Rhyfel Iron Age hillfort. 500 metres above sea level, this pear-shaped hillfort offers panoramic views over the Cambrian Mountains and beyond. The defensive bank was created by quarrying stone and earth, forming a barrier in front of the quarry pit. There are two potential entrances. Near them, a natural outcrop was used as a chevaux-de-frise, a defensive structure featuring jagged rocks to discourage intruders. Similar hillforts suggest that wooden huts for domestic or communal purposes may have once stood within Castell Rhyfel.
The valley contains stone round barrows, or burial mounds, from the Bronze Age, 4,300-2,700 years ago. One disturbed mound reveals a well-preserved, rectangular cist, or stone-lined box, used for burials. During the medieval and post-medieval periods, it became popular to graze animals here and shepherds’ huts and longhouses were built. Different architectural styles suggest that these were designed to suit owners’ needs. They were built using nearby stones, even using some from Bronze Age burial mounds. This period also saw the construction and improvement of routeways.
Animal grazing continued for hundreds of years, but changes did occur. Farms surrounding the valley took over land, reducing the amount of common land for others to use. Population growth, a decline in agriculture, and rural poverty drove poorer individuals to illegally construct houses, known as tai-unnos, Welsh for one night house, on the common land. This practice caused tension within communities. Traces of these buildings can be found today.