The city states that formed the basic administrative structure of Roman Britain.
As the Roman Legions established military control over Britain in the first century AD, they established for each tribe a civitas, an independent administrative unit based on the pre-existing Celtic tribal territories (or to be precise on where the Roman authorities understood the tribal territories to be, or should be) each with an administrative centre, or tribal capital, a town laid out on formal Roman lines, laden with the usual Roman civic amenities.
The idea being that the inhabitants could retain their old tribal identity within the civitas whilst gradually becoming assimilated to Roman ways. The existing Celtic tribal leaders were encouraged to administer the civitas, through the standard Roman structure of government with its elected council or ordo and magistrates, but always, of course, under the supervision of the Roman provincial administration based in Londinium.
By this method the Roman Empire could transform potential enemies into allies.