Information from the Gwynedd Archaeological Trust RHER:
It [Criccieth Castle] stands within an enclosure formed by the cliffs to the south and an earthen bank to the north which has the appearance of an Iron Age hillfort.
An urn discovered behind the cottage of the Castle custodian might suggest an Iron Age site. The find occurred at a depth of 2.6m behind the cottage of the Criccieth Castle custodian, and it is suggested that the bank under which it lay is a remnant of an Iron Age earthwork.
The rocky outcrop to the northwest of the castle is called Dinas, although there's no record of any site or finds there.