The town of Visby is situated on the island of Gotland south-east of Stockholm, reachable via small air-crafts in 1 hour (or a little bit longer via ferry) from the mainland.
The 21000 inhabitant town impresses with it's 3,5 km long limestone wall from 13th and 15th century including 44 towers. 13 ruins of churches, the still existing cathedral of St.Maria and the medieval touch, alleys, 200 medieval buildings and cosy cafés should be mentioned as well.
Visby, long time oriented towards Northern German Hanse mercants, became, as the whole Island, 1645 after Danish ruling finally Swedish. By the way - the wall was not built to protect against enemies, but against rich farmers from the islands itself, as the town civilians didn't want to share the Hanse trade priviledges with anyone else.
An attraction is also the medival festival during the second week of August, which I didn't have the chance to see yet. Visby's core was set under protection of the UNESCO in 1995.
However, Visby is flooded with tourists (many Germans!) every summer, what stands in tremendous contradiction to the sight the town provides during winter months, where you get the feeling of a lost world (can also be very interesting). The best time for my feeling is in late spring, when the green comes out attracted by the first warm sun (and when tourists haven't entered the stage yet).
Gotland itself - the largest Island in the baltic sea with 3140 km² - is worth some days - at least what I have seen up to now really looks like a picture of what tourists expect from Sweden. The sea view and the surrounding with bizarre water shaped rocks at the waterline compensate for the travel to the island.