Ar lethrau Mynydd Ystum, ryw 3km i'r gogledd o Aberdaron, mae caer fynyddig Castell Odo, sy'n nodedig am mai hi efallai, yw'r sefydliad cynharaf yng Nghymru o gyfnod yr Oes Haearn.

Mae Castell Odo ac ambell i safle fychain arall yn awgrymu symudiadau pellach i mewn i'r tir, oddi wrth eu safleoedd gwreiddiol ar yr arfordir.

On the slopes of Mynydd Ystym, about 3km to the north of Aberdaron, are the remains of Castell Odo. A site that has been partly excavated twice, that sheds light on the early inhabitants of Llyn.

Castell odo and a few other small sites also suggest further inland insurgence into Llyn by Iron age 'B' people.