Bomarsund is a nineteenth century fortress in Sund on the Åland Islands in the Baltic Sea . It was built in 1832 by Russia but destroyed twenty two years later in 1854 in the Crimean War by a British - French fleet. The Battle of Bomarsund was fought between the Russian defenses at Bomarsund and an Anglo-French task force. After a week of fighting the British stormed the "remaining" fort and at the end of the battle the fort of Bomarsund was destroyed. Three hundred Finnish grenadiers defending the fortress were captured and taken to a prison in Lewes , United Kingdom. They were later allowed to return to Finland, and they returned with a song telling about their battle and imprisonment, called the War of Åland (Finnish: Oolannin sota ). In the Treaty of Paris 1856, the entire Åland Islands were demilitarized, which is a status that has been preserved until this day.