How did Roman client kingdoms help protect the frontier in Britain?

Roman client kingdoms played a strategic role in protecting the frontier in Britain by serving as buffer states that spared Rome the expense and logistical burden of maintaining direct military defenses across all territories. These kingdoms were dependent on the emperor's support and were obligated to provide military aid to Rome when necessary.1

A primary example of this relationship was the kingdom of the Brigantes, led by Queen Cartimandua. Her territory, which spanned the region between Derbyshire and the Tyne, acted as a secure right flank for Roman forces. By maintaining a treaty relationship with her, Roman governors were able to concentrate their military resources on the conquest of [Wales]. Cartimandua’s cooperation—most notably her surrender of the resistance leader Caratacus to the Romans—demonstrates how these alliances directly supported Roman security objectives.2

Additionally, the Atrebates kingdom in the south served as a stable ally, and its restoration under Roman influence provided a base for military operations, such as those led by Vespasian.2