Rajhenburg Castle, also known as Brestanica Castle, stands in Brestanica above the confluence of the Brestanica and Sava rivers. The castle was certainly built very early, but the document from 29 September 895, which was long considered the first mention of the castle, and which made it the oldest castle in Slovenia, was found to be a forgery in research. This castle was destroyed during the Hungarian invasions of the Posavje region. After 1126, the Salzburg Archbishop Konrad I rebuilt the castle, which was first mentioned as "Reichenburch" in 1309. Brestanica Castle is an immovable monument of national importance and was declared so on 6 October 1999. A cultural national monument is a building that has exceptional cultural value at the local level. Such buildings are initially assessed as cultural heritage and then entered into the Register of Immovable Cultural Heritage. The declaration is carried out by acts of proclamation adopted by the competent authority. These acts are published in the Official Gazette of the Republic of Slovenia or the local community bulletin.
During the war, it was a center for Slovenian exiles, after the war it was a women's prison, and today it houses the Museum of Slovenian Exiles. Upon denationalization, the Trappists left the castle and estates to the Slovenian Roman Catholic Church, and even before the war they had given estates to many residents of Brestanica. The municipality of Krško invested 5 million euros in the renovation of the castle. The castle was ready for visitors by the end of 2012.