Suomenlinna is a UNESCO-listed sea fortress spread across a cluster of islands guarding the entrance to Helsinki, reached by a short ferry ride from the market square. Founded in the 18th century, its ramparts, tunnels, cannons and cobbled lanes make for a wonderful day out, alongside museums, cafes and the working community that lives there. Entry to the islands is open, while the ticketed museums and submarine bring its layered history to life. It is one of the capital’s essential sights.