Prison Ships and Fortress Prisons During the 18th and 19th Centuries - Swedish History Online Newspaper

The thesis Abysses of Misery compares English prison ships in Portsmouth with fortress prisons on the Swedish west coast during the late 18th century and up to the mid-19th century. The prisons in question were not purpose-built and over thirty prisoners were kept in terrible conditions in large prison cells, either in the vaults of the fortresses or below deck on the ships. Source