 Mynydd y Dref is the remains of an ancient volcano that erupted about 450 million years ago.At the summit of Conwy Mountain are the Neolithic Hut Circles and the Iron Age hillfort of Castell Caer Seion (sometimes called Castell Caer Lleion).Castell Caer Seion comprised a stone walled fort, and remains show this to have been an extensive site, incorporated more than 50 hut circles and levelled platform houses, and with a citadel and outposts.