Zagreb Cathedral on Kaptol is probably the most famous building in Zagreb, as its spires can be seen from many locations in the city. The building of the cathedral started in the 11th century (1093), although the building was razed to the ground by the Tatars in 1242. At the end of the 15th century, the Ottoman Empire invaded Bosnia and Croatia, triggering the construction of fortification walls around the cathedral. Few of these fortifications are still intact. In the 17th century, a fortified renaissance watchtower was erected on the south side, and was used as a military observation point, because of the Ottoman threat. In 1880, the cathedral was severely damaged in an earthquake. The main nave collapsed and the tower was damaged beyond repair. The restoration of the cathedral in the neogothic style was made by Hermann Bollé, bringing the cathedral to its present form. As part of that restoration, two spires of 105 m height were raised on the western side, both of which are in the process of being restored during a massive general restoration of the cathedral.