The ensemble of the Senate square is decorated by the symbol of Helsinki Lutheran Cathedral, the first Cathedral of St. Nicholas. Over the Church dominates the Central dome and sculptures of the 12 apostles. This building began Engel, a German architect, Ernst, Lorman, has added its four towers with domes and Golden crosses. Outbuildings are not very in tune with the General strict style of the building. Inside the room, decorated in a Protestant ascetic, attract the attention of the statue of Luther, Melanchthon and Mikael Agricola.

During the construction of the Cathedral at the bottom formed a high cruciform crypt. At the present time, the vault is converted into a space for exhibitions, concerts and meetings. Summer in the crypt houses a cozy cafe.