 Its highest points are at Allt Lom and at Twyn y Gaer trig point overlooking the valley of the River Usk.Twyn y Gaer is the site of an Iron Age hill fort.Like Cefn Llechid and Fforest Fach to the west, Mynydd Illtud is formed from the mudstones and sandstones of the St. Maughans Formation of the Old Red Sandstone laid down during the Devonian period.