Casa Batllo

Casa Batlló is the most photographed building on Passeig de Gràcia, which is saying something on a street that's an open-air Modernisme museum. Gaudí didn't build it from scratch — Josep Batlló, a textile magnate, hired him in 1904 to renovate a fairly ordinary 1877 apartment block. Gaudí kept the original structure and reimagined the surfaces.

The facade is the most famous element. Locals call it the Casa dels Ossos — house of bones — because of the skeletal balconies and the colonnaded ground floor that looks like vertebrae. The trencadís mosaic shimmers in different colours depending on the time of day. The roofline is a scaled dragon's back; the small tower with a cross is supposed to be St George's lance piercing the beast. Catalan dragon symbolism end to end.

The interior is the real surprise. Gaudí designed it as a single continuous space without right angles — doors arch, walls curve, the famous central light-well shifts from white tile at the bottom to deep blue at the top so light disperses evenly through every floor regardless of season. The attic with its catenary arches feels like the ribcage of a large mammal. The rooftop terrace, with its chimney pots that look like ice-cream cones in mosaic, is what most people remember.

Tickets are timed and tiered (the cheapest entry skips parts of the building; the most expensive includes the rooftop and a tablet "augmented reality" guide). Buy ahead — Casa Batlló is one of Barcelona's most-visited attractions, second only to Sagrada Família and Park Güell. First slot of the day is much less crowded.