The distinctive Brick Gothic style of architecture so prevalent in the Hanseatic towns along the Baltic Sea was developed here and in nearby Wismar. Both are designated as World Heritage Sites by UNESCO. Their medieval footprints are largely intact and provide a showplace of elaborate and imaginative decoration using fired clay bricks. The heyday of these cities was from the 13th to the 15th centuries. Especially noteworthy examples are the Nikolaikirche, which also holds a treasury of medieval artworks on its interior, and the dazzling Rathaus (Town Hall.) Another interesting local attraction is the Ozeanum, a combination aquarium and maritime museum focused largely on the Baltic region.