The late Bronze Age hillfort of Castell Odo on the Lleyn peninsula is one of the most important archaeological sites in Wales. It is situated on the summit of Mynydd Ystum and was probably built and colonised by Celtic settlers coming from the Irish Sea in around 400 BC. Its early construction was defensive in nature, but later phases of construction placed less emphasis on defence, and the ramparts were even flattened deliberately in the last phase of its development.
The site has been excavated twice, once in 1929 and again in 1958/59. The hill fort is enclosed by a weak double bank, with an entrance to the north-east of the structure. Inside the fort are indications that there were once circular buildings constructed of timber.