Castell Caer Seion, Snowdonia National Park

Castell Caer Seion is an Iron Age hillfort situated at the top of Conwy Mountain, in Conwy County, North Wales. It is unusual for the fact that the main fort contains a smaller, more heavily defended fort, complete with its own distinct defences and entrance, with no obvious means of access between the two. The construction date of the original fort is still unknown, but recent excavations have revealed evidence of occupation as early as the 6th century BC, whilst the smaller fort can be dated with reasonable certainty to around the 4th century BC. Whilst the forts were constructed in different periods, archaeologists have uncovered evidence of concurrent occupation, seemingly up until around the 2nd century BC. The larger fort contained around 50 roundhouses during its lifetime, whereas examinations of the smaller fort have turned up no more than six. The site was traditionally associated with Maelgwyn Gwynedd, but there is no evidence pointing to a 6th-century occupation. The fort and wider area beyond its boundaries have been said to retain significant archaeological potential, and are protected by law as a scheduled ancient monument. ()