At the start of the Middle Ages, England was a part of Britannia, a former province of the Roman Empire. The local economy had once been dominated by imperial Roman spending on a large military establishment, which in turn helped to support a complex network of towns, roads, and villas. At the end of the 4th century, however, Roman forces had been largely withdrawn, and this economy collapsed. Germanic immigrants began to arrive in increasing numbers during the 5th and 6th centuries, establishing small farms and settlements, and their language, Old English, swiftly spread as more settlers arrived and those of the previous inhabitants who had not moved west or to Brittany switched from British Celtic and British Latin to the migrants' language. New political and social identities emerged, including an Anglian culture in the east of England and a Saxon culture in the south, with local groups establishing "regiones", small polities ruled over by powerful families and individuals. By the 7th century, some rulers, including those of Wessex, East Anglia, Essex, and Kent, had begun to term themselves kings, living in "villae regales", royal centres, and collecting tribute from the surrounding "regiones"; these kingdoms are often referred to as the Heptarchy.