Suomenlinna is a sea fortress and UNESCO World Heritage site accessible by a short ferry ride from Helsinki. It offers a glimpse into Finnish history with its well-preserved ramparts, museums, and tunnels. While some areas are wheelchair accessible, the uneven terrain can pose challenges. Solo exploration is easily manageable, but guided tours offer deeper insights. The fortress is open year-round, but some museums and services have seasonal hours. Expect to spend at least half a day exploring. The ferry costs a few euros, and museum entrance fees vary. Consider bringing layers as the island can be windy and cool, even in summer.