The Limes Germanicus (Latin for Germanic frontier) was a line of frontier (limes) fortifications that bounded the ancient Roman provinces of Germania Inferior, Germania Superior and Raetia, dividing the Roman Empire and the unsubdued Germanic tribes from the years 83 to about 260 AD. At its height, the limes stretched from the North Sea outlet of the Rhine to near Regensburg (Castra Regina) on the Danube. Those two major rivers afforded natural protection from mass incursions into imperial territory, with the exception of a gap stretching roughly from Mogontiacum (Mainz) on the Rhine to Castra Regina.