Hammersta castle was a 13x18m wide two-storey stone building built around the year 1300 by Eringilse Nilsson, who's father was married to a sister of Bridget of Sweden. Eringilse's son (with the same name), was married to a Danish woman, Brita Olofsdotter Tott, and when the situation between Sweden and Denmark became tense around 1450, Tott was sentenced to death for spying for the Danes. Luckily for Brita she was pardoned and later paid for the famous fresco paintings that you can now see at the Ösmo Church.
The castle was abandoned in the 15th century. Between 1628 and 1767 the estate was owned by Horn, Kruus and Wrede families. The new manor house building was started in 1685 and completed in 1723.
References:Sigmaringen Castle was first mentioned in the year 1077 in the chronicles of Petershausen monastery. The oldest parts of the castle are concealed beneath the alterations made during the 17th and the 19th centuries. The secret of the earliest settlement built on this defendable rock will never be fully revealed: large-scale excavation work would be necessary, which the extensive land development renders impossible. Judging from the many Roman remains unearthed in the area around Sigmaringen, the 12th century keep known as the 'Roman Tower' could be traced back to a Roman predecessor.
The castle remains that have been preserved (gate, great hall and keep) date back to the Staufer period around 1200. The castle remains were integrated into subsequent buildings. The foundations of the castle buildings are to a large extent identical to the surrounding castle wall.
These remains give us a good idea of how the castle might have looked during the 12th century.