Suomenlinna

This sea fortress, built from 1748 on a group of islands off Helsinki harbour while Finland was part of Sweden, was constructed as a defence against Russia. It has been a UNESCO World Heritage Site since 1991. The islands are connected by bridges, and a visit takes you along ramparts, cannons, tunnels, and a rocky shoreline with views over the archipelago; around 800 people still live on the islands today. A city ferry runs to the fortress from Market Square throughout the year, and you should set aside a good part of the day for the visit.