Nastola Aurinkolinna (originally Salpausselä Sanatorium) is an Art Nouveau castle in Lahti , Nastola church village , designed by architect Wäinö Palmqvist in 1921 . It was originally built as a sanatorium for children with bone and joint tuberculosis . Aurinkolinna has also served as a military hospital, a hospital for children with central nervous system disabilities and for a while in the 1990s as a refugee reception center. The main building is currently being renovated for residential use and most of the building has already been taken into use as rental and owner-occupied dwellings.

The outer walls of the sun castle are made of at least five layers of solid brick, with a wall thickness of about 60-70 cm. The stone footing of the castle and the stairs on the southern façade, in turn, consist of large granite boulders. On the outside, the walls are plastered and painted yellow / white. The main building was the subject of a sr-1 protection order in 1997, which states that it is of great architectural and cultural-historical value. The windows of the main and outbuildings were renovated in 2014 under the supervision of the National Board of Antiquities, respecting the original frame division of the windows. Due to the renovation for residential use, the building technology of Aurinkolinna’s apartments is new in all respects. The property is heated by district heating, which is supported by a heat pump system that recovers heat from the exhaust air. The apartments have their own heat distribution centers with internal temperature measurement.