Constructed between 1886 and 1890 for aristocrat and industrialist Eusebi Güell, this was Antoni Gaudí’s second commission—and yet the architect’s budding originality was such that it already looks almost as if it were grown rather than built. The family quarters are conventional—“a normal Venetian palace,” a guide once sniffed on one of our tours—but the architect’s imagination ran wild above and below. The underground forest of brick columns and vaults in 10 musty cellars creates a honeycomb of stables and servants’ quarters, and functions as much as a root system as a foundation. The rooftop is even more startling, for Gaudí wrapped the chimneys with swirling abstract sculptures. Not only are they embedded with mosaics of broken pottery, they also employ artistic symbols from ancient Catalan tradition. After the restraint of the main residence, the rooftop is an exultation of the spirit.