Just a 15-20 minute walk from the Old Town of Tallinn, Estonia is Patarei Prison, which was in use from 1920 to 2002. After paying a small entrance fee (about 3 euros), you can wander the dark (and I mean dark) hallways, decrepit cells, infirmary, library, offices, execution rooms and other areas.

One of the prison’s hallways, Notice how it’s curved, this was a feature of the original fortress from the 1800’s, so a shot could not be fired straight a long distance.

This is the courtyard and up until World War II the prisoners could walk in open space here. After the start of the Soviet occupation, the open-roof cells to the left were built. Prisoners would be crammed in these small “boxes” for an hour of fresh air.
As can be seen from the images below, it’s as though the prisoners just up and left everything where it was, in its “natural” state. The fact that a visitor can wander through these buildings (which have no light other than the daylight coming through the windows) with no escorts is quite unusual.
A small guidebook is available at the entrance, and a few rooms are sign posted, but for the most part you’re left with your imagination.
Many prisoners were held here while awaiting trial. Hundreds of people were also executed at the prison, the last execution taking place in 1991. One of the rooms used for shooting people was later painted over in brown paint to hide the blood stains.

One of the execution rooms. Hangings and poisonings took place here. Note the floorboards in the center of the picture, these would allow for the person to fall enough distance to complete the hanging.
The prison faces the sea, and was originally a fortress built to protect the city of Tallinn in 1840. It’s been open as a Cultural Park since 2007 to preserve the memory of what life was like for some unfortunate people during the Soviet occupation.
For more information, visit: http://www.patarei.org.