The medieval town of Visby has been entered as a site of the UNESCO World heritage program. An impressive feature of Visby is the fortress wall that surrounds the city, dating from the time of the Hanseatic League. Christopher Polhem (1661-1751), the father of Swedish mechanical physics was born in Visby. He was also called the Archimedes of the north. The inhabitants of Gotland traditionally speak a distinct dialect of Swedish, known as Gutnish.