Union. With the accession of Gustav Vasa in 1523 and the establishment of royal power, the population of Stockholm began to grow, reaching 10,000 by 1600. The 17th century saw Sweden grow into a major European power, reflected in the development of the city of Stockholm. From 1610 to 1680 the population multiplied sixfold. In 1634, Stockholm became the official capital of the Swedish empire. Trading rules were also created that gave Stockholm an essential monopoly over trade between foreign merchants and other Swedish and Scandinavian territories. In 1697, Tre Kronor (castle) burned and was replaced by Stockholm Palace. Throughout Sweden's history, walls were created in Stockholm to defend the city from attacks. These defensive walls were modified throughout the 13th to the 16th century. In 1625, the Great Stockholm Fire of 1625 destroyed the southwestern section of Stadsholmen, an island in the centre of Stockholm. The amount of destruction led to the beginning of the demolition of the Stockholm walls. Today, most of the younger city walls cannot be found anywhere above ground. However, parts of the northern city walls are preserved in the Museum of Medieval Stockholm. In 1710, a plague killed about 20,000 (36 percent) of the population. After the end of the Great Northern War the city stagnated. Population growth halted and economic growth slowed. The city was in shock after having lost its place as the capital of a Great power. However, Stockholm maintained its role as the political centre of Sweden and continued to develop culturally under Gustav III. By the second half of the 19th century, Stockholm had regained its leading economic role. New industries emerged and Stockholm was transformed into an important trade and service centre as well as a key gateway point within Sweden. The population also grew dramatically during this time, mainly through immigration. At the end of the 19th century, less than 40% of the residents were Stockholm-born. Settlement began to expand outside the city limits. The 19th century saw the establishment of a number of scientific institutes, including the Karolinska Institutet. The General Art and Industrial Exposition was held in 1897. From 1887 to 1953 the Old Stockholm telephone tower was a landmark; originally built to link phone lines, it became redundant after these were buried, and it was later used for advertising. Stockholm became a modern, technologically advanced, and ethnically diverse city in the latter half of