The island’s eventful history can still be seen in many ways. Before becoming part of the Archipelago National Park, the island served as pasture for cattle and a military zone for a hundred years – first as a fortress of the Baltic Sea defence chain under Tsar Russian rule, and later as a base for Finland’s coastal artillery. In historical terms, the most valuable sites on the fortress island include the barracks and heavy artillery battery, fortifications on the northern tip of the island, the grenade launcher hill, the ruins of a 120 mm artillery battery and the cobbled road called “Pitkä ikävä” (= long and dreary).
Three artillery batteries, surrounded by ammunition storage facilities, artillery observation towers, barracks, residential buildings, canteens, food cellars, electricity centres, floodlight stations, animal shelters, wells, jetties and roads were built on the island at record speed. A narrow-track railway was constructed to manage the transport of heavy loads from the jetty by the northern shore of the island to the artillery batteries. Workers brought in from Asia toiled at the heavy excavation and building work. After Finland’s declaration of independence, the island was completely expropriated from the locals and Örö was handed over to the Finnish Defence Forces.
Conscripts and servicemen have been trained on this isolated fortress island since the 1920s. Intense development of the fortress began in the 1930s. Under the threat of war in 1939 extra military exercises were organised and a considerably higher number of military personnel were stationed at the fortress than before. During the war, approximately 500 military and civilian staff served on Örö. The island’s strategic position changed when the Hankoniemi peninsula was leased to the Soviet Union in 1940–1941. Örö was part of the Hanko front and the forces on the island participated in several war manoeuvres at the time. During the important Battle of Bengtskär in 1941, artillery on Örö provided support for the island of Bengtskär and the surrounding sea area.
Around 60 houses from various periods are preserved on Örö, alongside the fortifications built during the Russian reign. Of the three artillery batteries built by the Russians, two 12-inch Obuhov guns with casemates remain, plus four decommissioned 6-inch Canet guns and the ruins of a 120 mm artillery battery. One of the massive 12-inch guns has been restored as a museum gun and can be visited with a guide.