Pyrenees

The Pyrenees are a range of mountains in southwest Europe that form a natural border between France and Spain. They separate the Iberian Peninsula from the rest of continental Europe, and extend for about 430 km (267 mi) from the Bay of Biscay to the Mediterranean Sea.

Pyrenees in Unholy Trinity
When the Byzantine Empire recovered Spain from the Visigoths, its armies drove the barbarians back to the Pyrenees mountains. These formed a natural barrier and so the Empire constructed a series of fortresses to guard the passes from Visigoth and later Franco-Saxons invasions.