Casa Mila (La Pedrera), Barcelona

Casa Mila, Antoni Gaudi's 1912 stone quarry-inspired masterpiece, contains 32 interlocking apartments with floor plans where no two rooms are rectangular, challenging conventional architecture. Its undulating limestone facade and rooftop of twisted chimneys and warrior-helmet ventilation shafts define Barcelona's skyline. The building is 40% heavier than a typical stone structure of its size due to its innovative self-supporting stone arches, yet it uses 15% less iron reinforcement than the average building of the era. Its architectural audacity outperforms #8 Hearst Castle's traditional European influences, as Gaudi's organic forms directly inspired subsequent modernist movements. The property's annual maintenance runs at $2 million.