Kuressaare Fortress is a polyfunctional cultural and natural site, a museological introduction into the history and nature of Saare County, a venue of cultural and sport events and other outdoor activities. It is the most popular tourist destination on Saaremaa Island, with foreign tourist making up more than half its visitors. The state owned Saaremaa Museum, the biggest scholarly and cultural institution on the island, manages most of the territory of the site and its seat there. The most valuable part of the complex is its core a bishop's castle erected mainly in the 14th century in Late Gothic style (first mentioned in 1381). It is threestoried structure of the Konventshaus type built on a strictly quadrangular ground plan after models of European order architecture, with a powerful 7-story Defence Tower on the north corner and a more slender 6-story Watch Tower on the east corner. The building material, a local brand of limestone known as Saaremaa dolomite, is easy to work and has lent the building its characteristic decor. Surrounding the medieval castle is Kuressaare Fortress a 15th-17th century system of bastions, curtains, ravelins and moats. The original 15th-17th defence towers are now longer there; the biggest of them, the Cannon Tower on the North Bastion, was restored in the early 1970s and is now being used as the museum's depository. Four Classical stone houses surviving in their original appearance were erected in the territory of the fortress for the needs of its Russian garrison in the late 18th century. Kuressaare Fortress is very valuable as a natural site. The town park is a nature reserve with many plant species, one of the most beautiful in Estonia.