It lies on the territory of present-day Germany, Austria, Switzerland and Liechtenstein. As early as the period 15 BC to about 70 AD, the border between Rome and Germania ran mostly along the line of the late antique Danube-Iller-Rhine Limes (DIRL) before the Romans advanced further north into the "Agri decumates". Because of troop withdrawals and massive barbarian invasions, the Upper Germanic-Rhaetian Limes were abandoned in the late 3rd century and the Roman forces pulled the border back to the banks of the three rivers. Especially around the year 300, the Emperor Diocletian had new fortifications built directly on the river banks or on major roads in the hinterland here. Around 370 AD, the line of fortifications was considerably strengthened under the Emperor Valentinian I to counter the Alemanni, who were steadily advancing southwards. By contrast with the Upper Germanic-Rhaetian Limes, the DIRL primarily fulfilled defensive purposes; its camps had much stronger and higher walls than their High Imperial predecessors. Furthermore, they had in most cases been built to conform to the local topography. This meant that they could not be built in the classical 'pack of cards' shape. Between them a dense chain of watchtowers and signal towers was constructed to provide an additional security measure ("burgi").