The Danube–Iller–Rhine Limes or DIRL was a large-scale defensive system of the Roman Empire that was built after the project for the Upper Germanic-Rhaetian Limes in the late 3rd century AD. In a narrower sense the term refers only to the fortifications between Lake Constance and the River Danube ; in a broader sense it also includes the other Late Roman fortifications along the river Rhine on the High Rhine and on the Upper Rhine as well as the Upper Danube. Wikipedia