The Gop is a Neolithic mound, the second largest such structure in Britain being only superseded by Silbury Hill. Caer Drewyn is one of many Iron Age hill forts in Clwyd that attest to human occupation of this area for millennia. Maen Achwyfan Cross is a carved 10th century wheel cross depicting Celtic and Viking symbols. The castles of Rhuddlan and Flint were built by the Normans during their invasion of North Wales and Castell Dinas Brân was a Welsh fortress of the same period. St Asaph Cathedral also dates from the thirteenth century as does the medieval Cistercian abbey of Valle Crucis.