Alsnö hus (Swedish for "Alsnö House", where Alsnö is the old name for the Adelsö island) are the ruins of an old castle and a palace at the Hovgården settlement. It is located on Adelsö island, in Lake Mälaren in central-eastern Sweden. As part of Hovgården, it is declared a World Heritage Site.History
Next to the ruin is five burial mounds — the largest with a diameter of 45 metre — dating back to the Vendel Era (i.e. Late Iron Age, c. 500-800 CE) when Hovgården was a so called King's House (Kungsgård). The royal castle Alsnö hus reflects the importance of Birka, the trade settlement on Björkö island just south of Adelsö. However, Birka was abandoned around 975, but apparently the royal mansion continued to be of importance as the runestone U 11 (c. 1070 CE) was erected next to it.
Alsnö hus first appears in historical records in 1200 as mansionem regiam Alsnu, i.e. a house of more than ordinary proportions. However, 70 years later King Magnus Barnlock had the old fortress replaced by a summer residence built in brick. In contrast to the many fortifications built during the era, this building was a summer residence more adapted to offer comfort than defence and it regularly served both Magnus and his son Birger during summers. The Ordinance of Alsnö (Alsnö stadga) was made here in 1279, often said to be the foundation of the Swedish nobility as a separate social class and the start the Swedish feudal system. Large parts of the Romanesque parish church Adelsö Church next to the present ruins date from this period and was possibly commissioned by the king.
The building was ruined by the end of the 14th century, and little remains today. When the ruins were excavated in 1916-18, a huge number of crossbow arrows were found, indicating the palace might have been burnt down by the pirates of Albert von Mecklenburg who raided the Baltic in an attempt to reclaim the Swedish crown to Albert.
The ruins were bought by the Royal Swedish Academy of Letters, History and Antiquities (Vitterhetsakademien) in the 1950s and 1960s and is today preserved by the Swedish National Heritage Board (Riksantikvarieämbetet).