Palacio del Marqués de Salamanca en Vista Alegre
Time Out says
Facing the historic Plaza de Vistalegre is one of the prettiest palaces in Madrid, which, although it's been forgotten by a large part of the Administration, the neighbours want to claim as public space, although at the moment, unfortunately, their petitions have fallen on deaf ears. This palace complex, which was a Royal Site, as it was the summer residence of María Cristina de Borbón, dating from the time of Fernando VII, was a famous place of recreation for the Madrid aristocracy of the 19th century, with river and gardens included (you can see something similar in the Parque del Capricho, on the south side of the city). This was a favourite place of the daughters of Fernando VII, Isabel II and the Infanta Luisa Fernanda. Though it deteriorated quite a bit, still standing from that time is the Salón Árabe – the Arabian-style architecture had become fashionable in Europe at that time, inspired by the architecture in Alhambra. In those years of splendour, the building also housed many works of art, but in the late-19th century, it was passed into the hands of the State, which promised to use it for charity works. That's when the decline began: walls crumbled and works deteriorated in the abandonment, and it got to a point when no one could enjoy it. The palace has been emptied of its interior decoration (plaster included), and is waiting (eternally) for a plan as to what would be done with it, although there is something lined up for its remodelling. In addition to the palace of the Marquess of Salamanca (the new palace), the grounds are also home to the palace of María Cristina (the old palace) houses, which was once a school for orphans of La Unión, and is now a training centre for the faculty of the Comunidad de Madrid and other official buildings with no public access, surrounded by magnificent gardens (declared 'historical gardens'), which you can visit.