 To enter Northern Greece , the German army had to cross the Rhodope Mountains , which offered few river valleys or mountain passes capable of accommodating the movement of large military units . Two invasion courses were located west of Kyustendil ; another was along the Yugoslav @-@ Bulgarian border , via the Struma river valley to the south . Greek border fortifications had been adapted for the terrain and a formidable defence system covered the few available roads . The Struma and Nestos rivers cut across the mountain range along the Greek @-@ Bulgarian frontier and both of their valleys were protected by strong fortifications , as part of the larger Metaxas Line . This system of concrete pillboxes and field fortifications , constructed along the Bulgarian border in the late 1930s , was built on principles similar to those of the Maginot Line . Its strength resided mainly in the inaccessibility of the intermediate terrain leading up to the defence positions . 
