From around 450 BC, the territory of modern-day Slovakia was settled by Celts, who built powerful oppida in Bratislava and Havránok. Biatecs, the silver coins with the names of Celtic kings, represent the first known use of writing in Slovakia. From 6 AD, the expanding Roman Empire established and maintained a chain of outposts around the Danube. The Kingdom of Vannius, a barbarian kingdom founded by the Germanic tribe of Quadi, existed in western and central Slovakia from 20 to 50 AD.