It was built in the 12th century in stone, shaped as a cross with a tall central tower. It is located close to Mariakyrkan, the only Medieval church that survived the Middle Ages, and these two churches were part of a large complex that also included a Dominican Monastery.
King Olaf had some connection to Sigtuna. King Olaf Skötkonung made Sigtuna an important centre in his kingdom. Olaf Haraldsson wanted to marry Olaf Skötkonung's daughter Ingegerd, infuriating the Swedish king, as he was a sworn enemy to the Norwegian king. He is said to have called the Norwegian king "tjocken"- "fatso." Ingegerd was never made queen of Norway, but ended up married to Prince Yaroslav of Novgorod in Russia.
In spite of the bad blood between the two kings, a church dedicated to the "Eternal King of Norway" was built around 1100. There has been an even older church building within the present walls.
It fell into disrepair during th 17th century and works to secure the remains started in the late 19th century.
St Olofs Kyrka must have been a impressive sight, and still is, even as a ruin. Walking inside this once impressive building is a strange experience. It is absolutely worth a visit, even though the nearby Mariakyrkan gives a much better impression of what a Medieval church must have looked like.