Moel y Gaer (Welsh for "bald hill of the fortress") is a prehistoric hill fort located on a summit of Halkyn Mountain, overlooking the village of Rhosesmor, Flintshire, Wales.
Excavations suggest that the site was occupied, perhaps sporadically, from the Neolithic period to sometime before the Roman conquest of the area. Remains of a timber long house on the hill summit have been dated to the third millennium BC and may represent the earliest human occupation of the region. Defensive construction at the site began around the late seventh century BC, and at first consisted of a timber palisade enclosing a settlement of timber roundhouses. The palisade was later replaced by a stone-and-earth rampart and a ditch.
Much of the archaeological work at Moel y Gaer was carried out in the early 1970s, before the construction of a reservoir on the hilltop in 1979 destroyed a portion of the remains.