Types of civitates
As the empire grew, inhabitants of the outlying Roman provinces would either be classed as dediticii, meaning "capitulants", or be treated as client kingdoms with some independence guaranteed through treaties. There were three categories of autonomous native communities under Roman rule: the highest, civitates foederatae ("allied states"), were formed with formally independent and equal cities, and sealed by a common treaty (foedus); next came the civitates liberae ("free states"), which indicated communities that had been granted specific privileges by Rome, often in the form of tax immunity (hence liberae et immunes); the final, and by far most common group, were the civitates stipendariae ("tributary states"), which while retaining their internal legal autonomy were obliged to pay tax.
Prestigious and economically important settlements such as Massilia and Messana are examples of occupied regions granted semi-autonomy during the Roman Republic. The island of Malta was granted this status as a reward for loyalty to Rome during the Second Punic War. The new Romanised urban settlements of these client tribes were also called civitates and were usually re-founded close to the site of an old, pre-Roman capital. At Cirencester, for example, the Romans made use of the army base that originally oversaw the nearby tribal oppidum to create a civitas.
During the later empire, the term was applied not only to friendly native tribes and their towns but also to local government divisions in peaceful provinces that carried out civil administration. Land destined to become a civitas was officially divided up, some being granted to the locals and some being owned by the civil government. A basic street grid would be surveyed in but the development of the civitas from there was left to the inhabitants although occasional imperial grants for new public buildings would be made.
Tacitus describes how the Romanised Britons embraced the new urban centres:
They spoke of such novelties as 'civilisation', when this was really only a feature of their slavery (Agricola, 21)
The civitates differed from the less well-planned vici that grew up haphazardly around military garrisons; coloniae, which were settlements of retired troops; and municipia, formal political entities created from existing settlements. The civitates were regional market towns complete with a basilica and forum complex providing an administrative and economic focus. Civitates had a primary purpose of stimulating the local economy in order to raise taxes and produce raw materials. All this activity was administered by an ordo or curia, a civitas council consisting of men of sufficient social rank to be able to stand for public office.
Defensive measures were limited at the civitates, rarely more than palisaded earthworks in times of trouble, if even that. Towards the end of the empire, the civitates' own local militias, led by a decurion, likely served as the only defensive force in outlying Romanised areas threatened by barbarians. There is evidence that some civitates maintained some degree of Romanisation and served as population centres beyond the official Roman withdrawal, albeit with limited resources.
Certain civitates groups survived as distinct tribal groupings even beyond the fall of the Roman Empire, particularly in Britain and northern Spain.