The origins of Tallinn date back Viking era, but current walled city dates back to the 13th century, when a castle was built there by Danish King and the crusading knights of the Teutonic Order. It developed as a major centre of the Hanseatic League, and its wealth is demonstrated by the opulence of the public buildings and churches and the domestic architecture of the merchants’ houses, which have survived to a remarkable degree despite the ravages of fire and war in the intervening centuries. Due to its exceptionally intact 13th century city plan, the Old Town was inscribed as a UNESCO World Heritage Site in 1997, joining the ranks of the world’s most recognised landmarks. The Historic Centre of Tallinn is an exceptionally complete and well-preserved medieval northern European trading city on the coast of the Baltic Sea, surrounded by city wall of which 1,9 km has survived to this day