Alsnö hus, Medieval castle ruins in Adelsö socken, Sweden

Alsnö hus is the ruin of a medieval brick palace on Adelsö island in Lake Mälaren, of which only parts of the foundation walls remain on a grassy mound by the shore. The ruin sits next to five burial mounds up to 45 metres in diameter from the Vendel Period, and the Romanesque Adelsö Church stands nearby.

Alsnö hus is first recorded in 1200 as 'mansionem regiam Alsnu'. Around 1270 King Magnus Ladulås replaced it with a brick summer palace, where the Ordinance of Alsnö was issued in 1279. The building was ruined by the end of the 14th century, possibly burnt by Mecklenburg pirates; excavations in 1916–18 found many crossbow bolts.

The name Alsnö hus refers to the royal manor recorded in 1200 as mansionem regiam Alsnu. Here, at the herredag of 1280, Magnus Ladulås issued the Ordinance of Alsnö, which established the Swedish nobility as a separate estate. The site lies within the Hovgården archaeological area, beside five Vendel Period burial mounds up to 45 metres across.

The ruin lies on a grassy rise by the shore northeast of Adelsö Church, next to the Hovgården burial mounds. Visitors reach Adelsö by road from Ekerö and continue to the church area; from there a short walk leads to the stones. The blocks are low and partly hidden by tall grass, so watch your footing.

Excavations from 1916 to 1918 uncovered many crossbow bolts scattered throughout the ruins, suggesting a violent end. These findings indicate the palace likely fell to a deliberate attack rather than simple decay.