The oldest building in Zagreb was built around 1247. It is a property of the Zagreb Chapter located in  Gradec, around which many disputes arose between the Chapter and the city administration due to ownership of the tower itself. It was popularly named Pope’s Tower by investors. The earthquake in 1880 significantly damaged the building, in the process of reparation, the walls were connected by installing iron rods – tensioners that are still visible today. The Pope’s Tower underwent its last major change in 1903 when the tent-like roof was removed and a third floor was added, with a special arch and terrace to serve the Croatian Natural Science Society observatory. The work was entrusted in 1903 to the architect Vjekoslav Heinzel, and the observatory was opened on December 5, 1903. It was founded by the Croatian Natural Science Society, which at that time was called the Croatian Natural Science Society. The concept of the Observatory as a public institution, open to young people and citizens, ranks among the oldest and most successful observatories of this profile in Europe.