This 30 meters high rock hill was used as guard hill in the Middle Ages. If enemy ships were seen coming from the sea, fire was lighted to the hill to warn local people. After the great fire in Turku (1827), instruments and astronomical books of Turku University observatory were transferred to Helsinki.
The Helsinki astronomical observatory was built to the hill in 1834. It designed in cooperation by professor Friedrich Wilhelm Argelander and architect Carl Ludvig Engel. The observatory was among the most modern astronomical observatories of its time, and served as an example for several European observatories that were built afterwards. A separate tower was built in the observatory garden for the telescope designed for astrophotography. This building was finished in 1890.
At the top of the hill is is also situated Stigell’s monument Haaksirikkoiset (“shipwrecked”), erected in 1898. The sculpture is stretching to the west and increased the national spirit in the end of the 19th century.
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.