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:Visby Cathedral (also known as St. Mary’s Church) is the only survived medieval church in Visby. It was originally built for German merchants and inaugurated in 1225. Around the year 1350 the church was enlarged and converted into a basilica. The two-storey magazine was also added then above the nave as a warehouse for merchants.
Following the Reformation, the church was transformed into a parish church for the town of Visby. All other churches were abandoned. Shortly after the Reformation, in 1572, Gotland was made into its own Diocese, and the church designated its cathedral.
There is not much left of the original interior. The font is made of local red marble in the 13th century. The pulpit was made in Lübeck in 1684. There are 400 graves under the church floor.