Ilchester was settled by the Durotigians, a celtic tribe in the Iron Age from 600 BCE to the early part of the first century CE. Evidence of Round Houses has been found and a Durotrigian coin can be seen in the Museum.
The site of Ilchester overlies an outcrop of Jurassic limestone and limestone clay on top of which sits alluvial deposits and flint gravel. This raised the site above the winter marshes and floods making it a good place for a settlement.
The Durotrigian lands stretched from West Hampshire through parts of Wiltshire and South Somerset. The Iron age fort of Ham Hill lies to the south west of Ilchester, Compton Dundon to the north east and that of Cadbury Camp to the south east. They were on the borders of lands held by the Dobunni tribe to the north, the Belgiae to the east and the Dumnonii to the west.