Trail of la Guerra Civil del Frente del Agua Activity · Puentes Viejas, Sierra Norte, Madrid A circular route of some eight kilometres along a forest track at Paredes de Buitrago, with twenty-six restored features from the war — machine-gun nests, bunkers, trenches and command posts — that defended the reservoirs supplying Madrid. Photo: jacinta lluch valero from madrid * barcelona...., (España-Spain) (CC BY-SA 2.0) What to see and do in Puentes Viejas