county town
noun [ C ] UK uk/ˌkaʊn.ti ˈtaʊn/ us/ˌkaʊn.t̬i ˈtaʊn/ (US county seat)
（英国）郡首府，郡政府所在地;（美国）县首府，县政府所在地
the most important town or city in a county, especially the one where the local government is based
Cambridge is the county town of Cambridgeshire. 剑桥是剑桥郡的首府。