Svartholm Fortress guards an island at the mouth of Loviisa Bay, about ten kilometres from the town centre. It was built in the same 18th-century Swedish defence scheme as the land fortress, then battered and partly blown up during the Crimean War.
Restoration has made the island readable again without polishing away the ruin. In summer, boats from Loviisa make it the best excursion in town: sea air, granite, low walls, and a very clear sense of the Baltic frontier.