"Constructed in the 17th century but heavily modified a century later, this castle—actually more of a fortress—lords over Montjuïc hill. Its use as a military bastion spans from the 1600s to the mid-1800s, when Barcelona experienced a number of government insurgencies. It was later used as a prison and execution site for anarchists—most notoriously during the Franco regime, when the exiled president of Catalonia, Lluís Companys, was killed there by a firing squad in 1940. Today it’s a museum tracing the castle’s long military history. A platform atop the castle’s parade ground provides excellent views of the city and harbor below. Castle admission is free on Sundays after 3 p.m."