The plan governing the development of the limes was relatively simple. From a strategic point of view, the Agri Decumates, or region between the Rhine and Danube, offers a bulge in the line between the Celts and the Germanics, which the Germanics had tried to exploit under Ariovistus. The bulge divided the densely populated Celtic settlements along the entire river system in two. Invading forces could move up under cover of the Black Forest. Roman defensive works therefore cut across the base of the bulge, denying the protected corridor and shortening the line.