By 410 CE, the Roman Empire had crumbled and Britain, at the periphery of the Roman Empire, began to come under increased threat from marauding tribes whilst Rome weakened. Troops and money could not be spared to defend Britain from increasingly frequent raids by Picts and Scoti in the north and seaborne raids from Saxons and Angles. The system of coastal forts known as the Saxon Shore had been in use for over 100 years before the Romans left Britain completely in around 410 CE. The forts were left largely undefended after Constantine III took the remaining troops he could gather in Britain to Gaul in an effort to establish himself as the new Western Roman Emperor in 407 CE. After the Roman departure the Anglo-Saxons gradually formed separate tribal kingdoms and sub-kingdoms, introduced their own language (the basis for modern English), replaced the Romanic stone buildings with wooden ones and introduced their own gods. The Angles and Saxons originally acted as mercenaries to the Romano-British Celts, who were exposed to invasions from the northern Scotti and Picts. The mercenaries, angry about not being paid, turned into enemy invaders who invited further warriors to British shores and, some historians argue, their families. First to fall to the Angles and Saxons were the east and southern coasts, from where the Angles and Saxons pushed into the different Roman administrative areas, except for Secunda, western Prima and Valentia.