Tre'r Ceiri is a hillfort dating back to the Iron Age. The name means 'town of the giants', from cewri, plural of cawr, 'giant'.

It is one of the best preserved and most densely occupied hillforts in Britain, with stunning views across the Llŷn Peninsula in Gwynedd

The settlement is located 1,480 feet above sea level on the slopes of Yr Eifl, a mountain on the north coast of the Llŷn Peninsula in Gwynedd, north-western Wales.

The settlement was first built around 200 BC, though most of the archaeological finds date from AD 150–400, showing the site continued as a settlement during the Roman occupation. The settlement was abandoned in the 4th centruy AD.

Tre'r Ceiri is one of the most spectacular ancient monuments in Wales. The settlement is surrounded by stone walls that are largely intact, and which reach up to 13 feet in some places. Within the walls are ruins of about 150 stone round houses, which would have had turf roofs. During Roman times, it may have housed up to 400 people.

It is suggested that because the settlement is so far above sea level, the huts served as habitations for summer shepherds who also had winter dwellings in the lowlands.

The earliest fort was defined by a stone wall, which surrounded an elongate area of the ridge top. It enclosed an Early Bronze Age cairn.

The fort was entered through two main entrances, each with approaching trackways. A second outer wall was later built around the western and northern sides of the fort.
Small oval, terraced enclosures surround the fort and were probably used as stock enclosures and cultivation plots. A spring immediately outside the fort probably provided the water supply for people and animals.