Bilbao's Iron Ring
Bilbao's Iron Ring (Spanish: Cinturón de Hierro, Basque: Bilboko Burdin Hesia), was a vast, labyrinthine fortification around Bilbao, consisting of bunkers, tunnels, and fortified trenches in several rings, protected by artillery. It was hurriedly built during the Spanish Civil War by the Basque Government. However, it was quite an antiquated defence concept akin to First World War fortifications, so it was vulnerable to modern warfare weapons of its time such as aircraft and artillery, and only 30,000 troops were defending it (it was conceived to be defended by 70,000) — therefore the Iron Ring was rather easily overcome by the Nationalist faction forces in the battle of Bilbao.
Coordinates: 39°56′N 0°34′W / 39.933°N 0.567°W
...
Wikipedia