The city's most prominent park, was created in the late 17th century as a buen retiro ("pleasant retreat") for the royal court and since 1868 a public park, this 120-hectare (297-acre) spread is anchored by a manmade water basin presided by a 1922 colonnade and equestrian statue of late-19th-century king Alfonso XII (these days it's plied by rentable dinghies). Notable buildings in the park include the Palacio Velázquez and Palacio de Cristal, both built in the 1880s, as well as the 17th-century Salón de Reinos ("Hall of Kingdoms"), a remnant of the original Buen Retiro royal palace; all are used these days for temporary exhibitions. Meanwhile, out amid the woodsy and grassy patches interspersed with smaller ponds you'll find plantings including a rose garden. Among the statuary, one especially curious standout is possibly the world's only public statue of Lucifer, the 1878 Fountain of the Fallen Angel. Most days and especially on weekends, you can find both locals and visitors strolling, biking, sunbathing, and sitting in outdoor cafés.
Read more in my post Landscape of Light: Celebrating Madrid's Very First UNESCO World Heritage Site.