Next door to Casa Amatller, Casa Batlló was designed by Gaudí in 1905. The facade’s sinuous curves give the structure a lush organic appeal, and the balconies, like those at La Pedrera, seem to be sculpted ocean waves. Floral references in the ornamentation turn more faunalike as the building rises, with the roof evoking the scaly skin of a dragon. Touring means climbing the spiral staircase around the central light shaft and starting with the Batlló family quarters, where Gaudí’s architectural flourishes and his furniture designs vie for your attention. Señor Batlló’s office has a little nook with two benches and a stove for warmth—perfect for a courting couple to sit on one side and their chaperone on the other. Even this smallest of the rooms has a skylight to let in natural light. All the decor, including waterlike eddies in the swirling ceiling, allude to the marine world; in the sewing room, which overlooks an interior courtyard, an ingenious ventilation system of sliding slats seems inspired by fish gills. The roof terrace has chimneys designed to evoke the backbone of the dragon slain by Sant Jordi (St. George), patron saint of Catalunya. It’s not the cheapest admission in town, but a cool new interactive program makes the high price almost worth it: As you walk through the house with a smart phone, an app deletes the other tourists from your view and reinstates original furnishings, even “lighting” a fire in the fireplace. Fun! Gawkers crowd the sidewalk in front of the eccentric building, and lines for admission can get quite long; it’s smart to get your ticket online ahead of time, which will save you 4€; for an extra 5€ you can zip to the front of the line. The outdoor terrace has live music with cocktails from mid-May through October.