 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.
