 The Gop is a Neolithic mound , the second largest such structure in Britain being only superseded by Silbury Hill .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 the county 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 . 
