Set in an unpromising spot underneath an overpass on the busy Paseo de Castellana, this open-air sculpture park has 17 pieces by some of Spain’s most prominent abstract artists. When the elevated roadway was built in 1970, the artist Eusebio Sempere—who had been commissioned to design its hand railings—had the idea for the park. Bearing in mind that Spain was still in the grip of Franco’s arch conservatism, it was a bold idea. The works represent two generations of the Spanish avant-garde including Joan Miró, Eduardo Chillada, and Sempere himself, whose waterfall of alternating wave forms is a highlight. In this location, you might expect it to be dark, dangerous, and graffiti-daubed, but somehow, it’s not.