The mountain pass of Rupel
is much known from the two world wars. It is formed between the mountains of
Agistro and the eastern footholds of Beles (through this mountain pass,
Strimonas river flows and near its north exit there is the Greek-Bulgarian frontier).
Due to its strategic importance after the Second Balkan War, an interceptive system of fortresses was built from which the strongest is the one of Rupel that is found in the northern exit of the mountain pass.
It is also worth reading about the byzantine castle in Sidirokastro.