Banco de España
Believe it or not, this huge building in the Paseo del Prado that isn't much to look at outside is one of Madrid's most beautiful on the inside. Originally the Palace of the Marquis of Alcañices, this Banco de España site was declared a Bien de Interés Cultural (Site of Cultural Interest) in 1999, and, after several additions and alterations, became the huge complex it is today. The interior patio, with a 27-metre-high ceiling, has more modern and Art Deco elements, including an impressive top window and a large clock at the centre of the patio. It's worth stopping in just to appreciate this marvel.