Following the war of 1808–09, Russia began building this major military structure as its westernmost defence against the Swedes. The fortress was still incomplete when the Crimean War began in 1854, and a French-British naval force bombarded it heavily from the sea. Within four days the Russians were forced to surrender it.

　　The evocative ruins stretch for a couple of kilometres, straddling the road and overlooking the sea. Across the water on Prästö, the small Bomarsund Museum displays excavated artefacts.

　　At the complex's core was a huge fortress, built from brick and strengthened with distinctive octagonal blocks, containing a garrison town, and protected by ramparts and a planned 15 fortified towers. Prästö became Bomarsund’s island of the dead, with a military hospital and separate Greek Orthodox, Jewish, Muslim and Christian graveyards. The epic construction drew masons, craftsmen and soldiers from across the Russian Empire.

　　In the Huvudfästet (main fort), only three of the defensive towers were completed. Today, they are an impressive sight, particularly Brännklint tower, its walls scarred by cannon and rifle fire. The overgrown foundations of the garrison town Nya Skarpans, populated now only by ants and butterflies, are also atmospheric.