Dinas Dinlle is an Iron age hillfort, and possibly once the site of a Roman lighthouse, located southwest of Caernarfon on the Irish sea in North Wales. The hillfort is a roughly rectangular defended enclosure with a high inner rampart and also an outer terraced rampart, which is divided by a deep ditch. It measures approx. 230m north-south by 145m east-west and would originally have enclosed approx. 4 hectares.
The fortifications survive on only three sides today as the west side has been eroded away by the sea. Formerly the site may have stood well inland.
Even today, it is possible to make out small depressions which are thought to indicate the locations of Iron Age huts. It also includes a mound that may be the remains of a barrow or burial. Finds of Roman pottery suggest that the Romans occupied the site in the 2nd or 3rd centuries AD, when they would have built their lighthouse.