Situated 10km offshore from Loviisa, this four-bastion fortress was built in 1748 to protect against further Russian invasion after Swedish losses in eastern Finland. It lasted until the Crimean War (1853–56), when it was largely destroyed by the British. The reconstructed fortress is a fabulous place to explore, and you can learn more at the free museum. There's an on-site cafe.
A waterbus runs from Laivasilta Marina (adult/child return €18/9, 35 minutes; cash only). Alternatively, the beautiful replica 19th-century yacht Österstjernan makes occasional trips.