All four bastions of the fortress have been similar. The walls were 7–9 metres high and each bastion had 12 vaulted casemates. There could be placed for instance cannons.
While the fortress was built it was thought that each bastion could as needed work as a fortress within the fortress. The bastions therefore had food and ammunition, and water was supplied through small holes in the walls. Grey constructions high up lead rain water away from the walls. The idea was to prevent the walls from crumbling. The building in the inner yard belonged to the engineers’ office.
Svartholma was a prison both during the Swedish and the Russian time. The prisoners were men, all from murderers to thieves and vagrants. Vagrants were people who did not have jobs. They had to stay in prison until they found work. In practice this often meant long sentences.
In Svartholma the prisoners were put for instance in casemates and the conditions were in no way good. The casemates were dark, cold and moist. The prisoners slept on straws and one casemate could hold up to twenty prisoners. Feeding was during the 18th and 19th centuries organised in prisons in a different way than nowadays. During the Swedish time a system was adopted, according to which the prisoners were given a daily food money allowance of a given size. With this money they could arrange for their food in practice usually through the prison guards or the janitor.
The work of the prisoners was not as heavy as in the other fortresses. The daily work could include preparing food, cleaning the fortress, chimney sweeping, emptying the latrine, clearing snow and cleaning and airing the prisoners’ rooms. According to preserved documents the prisoners, guards and soldiers of the fortress at times drank heavily together.
The prisoners were allowed to have own pastime works. In Svartholma the prisoners manufactured different kinds of furniture and graceful storage boxes. The boxes were often decorated with decorations made out of rye straws on red or dark brown base. The prisoners also did various straw works and an organ was also made at the fortress for the own congregation. The organ was later moved to the church of Ruotsinpyhtää.
The prisoners were sent to Suomenlinna in 1847. However, new prisoners were still brought to Svartholma and the prison functions did not end until 1853, when the entire fortress was emptied during the Crimean War. The prisoners were at that time taken to Hämeenlinna.
There have also been Russian political prisoners at Svartholma, seven decabrists. They were placed in a secret prison outside the main gate. The building was an earlier guardhouse made of wood and it had five small rooms with earthen floors. Apparently the conditions were not considerably better than for the Finnish prisoners. This can be concluded from the poem written by poet and artillery officer Gavril Stepanovich Batenkov in May 1827 who as a prisoner described the despair and monotony caused by the gloom of the prison. It is said that the decabrists later were let to choose if they would remain at Svartholma or if they were to be banished to Siberia. Most of them chose Siberia.
