This swirling, melting wedding cake of a building is as close as Madrid comes to the work of Antoni Gaudí, which so illuminates Barcelona. It’s a joyously self-indulgent (if slightly grimy) ode to Modernisme (an architectural and artistic style, influenced by art nouveau and sometimes known as Catalan modernism) and is virtually one of a kind in Madrid. Casual visitors are actively discouraged, but what you see from the street is impressive enough.

　　The only exception is on the first Monday of October, International Architecture Day, when its interior staircase alone is reason enough to come and look inside.