The Iron Age in north-eastern Thailand is marked by the appearance of multivallate ‘moated sites’ some of them up to 50ha in extent. Current evidence for their date and function shows them to be contemporary with other developments – expansion into new agricultural land, increases of ranking in burial and the arrival of regional pottery industries. In interpreting the reasons for these changes, the author draws on analogies from the Bronze/Iron transition in Britain, where forts are also seen as instruments of socio-economic change.