Caretimandua was queen of the Brigantes tribe of northern Britain at the time of the Roman invasion in 43AD. Cartimandua initially supported the Romans in exchange for being allowed to maintain control of her lands - a practice known as "client-kingship".
In 51 AD Caratacus, leader of the British resistance to the Romans, fled to Cartimandua for support. However, she betrayed him to the Romans, thus depriving Celtic Britain of its most influential and steadfast military leader.
This was a fairly typical act for the Celts, who could never stop fighting amongst themselves long enough to mount a serious obstacle to the Roman advance. But it also indicates the extent to which Cartimandua owed her continued power to her alliance with the Romans.
Cartimandua soon had cause to be grateful to her Roman allies; in 57 AD her husband Venutius tried to oust her and sieze power for himself, but the Romans put down the rebellion. The couple were reconciled for a time and ruled jointly until 69 AD, when Cartimandua deserted Venetius for his armour-bearer, Vellocatus. She fled Brigantine territory and was never heard from again.