Caer Seion is a large and strong hill fort. Excavations were carried out in 1951 but did not produce any dating evidence. However, it is believed to have been occupied during the Middle to Late Iron Age, about 300 BC to 1st century AD and it seemed likely that the fort was abandoned when the Romans conquered North Wales in 78 AD. The fort had a commanding position overlooking Conwy Bay and estuary and over the ancient trackway that followed the coastal ridge, continuing westwards towards Anglesey. The next nearest strong forts and possibly tribal centres are at Pen-y-dinas on the Great Orme, Pen-y-gaer above Llanbedrycennin and Braich y Dinas above Penmaenmawr.
The fort had two periods of defence. In the first period a single stone wall of 3 to 4m width encircled the whole hill top apart from at the steep north side where no defence was needed. It had one entrance, at the south side, which would have had a timber gate. The greater width of the wall around the gate suggests it had a ‘fighting tower’ over it. Within the fort were over 50 timber round houses, many just behind the rampart at the south side (to shelter from the wind) and they are visible as circular platforms terraced into the hill slope. They varied in size from about 4m to 8m diameter and traces of walling survive at some of them. In one place there is a possible corn-drying kiln.