In 1945 the Soviet occupying forces turned the former commercial kitchen compound into an internment camp. The cellar was converted by the prisoners into a remand center. Victims reported that they were tormented by sleep deprivation, beatings, kickings, being forced to stand for hours or subjected to water torture. Food, clothing, and hygiene standards were terrible. Some 1,000 people died.

In 1951 the newly-formed East German secret state police, the Stasi, took over the prison. During the 50s most inmates were those opposed to the communist dictatorship, such as reformers and strike leaders involved in the 17 June 1953 uprising. As there was never any daylight in the damp cells, the inmates nick-named the prison 'U-Boot," German for submarine.

At the end of the 50s a new building with more than 200 cells and interrogation rooms replaced the old cellar jail. Physical violence was replaced by psychological torture. After the Berlin Wall was built in 1961 most inmates were those who had attempted to escape or leave East Germany, but also writers and civil rights activists.

In the 70s most prisoners were brought through the city to the jail in Hohenschönhausen in these Barkas B 1000 vehicles. Made to appear outwardly as fish or vegetable delivery vans, these vehicles had five tiny windowless cells, which meant inmates had no idea where they had been taken. The Stasi succeeded in pressuring 90 percent of inmates to make damning statements in their first interrogation.

In prison every inmate was addressed not by name but by their cell number. To socially ostracize them they were often put into isolation cells for months, where even talking to the guards was forbidden. The only human contact was therefore with the interrogator - an insidious way to make inmates talk.

Up to three inmates were housed in the different sized cells. They were unable to see anything through the cell windows, which were made of glass blocks. A mirror and hot water was only made available as of 1983. During the day inmates were not allowed to lie on their cots, at night they had to assume the same position: lying on their backs, facing the door with their hands on top of the blanket.

Inmates found being permanently watched in their cells through the spy hole in the door very stressful. Guards would keep a check on the prisoners even when they were washing or using the toilet. At night the lights would be switched on every ten to twelve minutes. Heating and light could only be controlled from outside the cells. This all served to make the inmates feel utterly powerless.

A wire was mounted along the walls of the cell block corridor. When a prisoner was taken from his cell to be interrogated, the guards pulled the wire, which made red warning lights light up. Any inmate in the corridor would then have to face the wall immediately. This was intended to prevent prisoners encountering one another.

The cell block and the interrogation rooms where separated by barred doors. To this day the linoleum floor still smells of the disinfectant used in East Germany. All 120 interrogation rooms were equipped with double padded doors, behind which inmates where subjected to hours of questioning over several months. Prisoners were expected to incriminate themselves so that they could be sentenced.

Stasi police used elaborate psychological interrogation methods. Initially they would threaten the inmate with long prison terms or the arrest of their family members. Panic and uncertainty were intended to wear them down. Those who cooperated were promised an easing of detention conditions: medical attention, a book, or half an hour of yard exercise.

In these cell-like compounds inmates could see the sky and breathe some fresh air. They themselves called the yards "tiger cages." It was forbidden to talk, sing, stop, or to go anywhere near the four-meter (4.1-yard) prison wall. An armed guard was always on patrol above the wire mesh.

The fall of the Berlin Wall put an end to the Stasi remand center. But only few interrogators were ever made accountable for what had happened behind these walls, and none were sent to jail. As the prison buildings and the interior survived unharmed, today's memorial site of Hohenschönhausen gives an authentic insight into the former East German justice system.

The Berlin Wall divided West and East Berlin for 28 years. Since German reunification in 1990, the city is finally whole again. Still, one of the best ways to explore the last traces of this Cold War past is to cycle or hike along the Berlin Wall Trail. The 160-kilometer tour is well signposted.

The tour can easily be started anywhere you want. An interesting place to begin is the Berlin Wall Memorial. Following the wall's former location on Bernauer Street, it shows how the border fortifications were set up and pays tribute to those who fled or died trying to escape East Berlin.

These cobblestones will help you recognize where the Berlin Wall once stood. However, only certain parts of the former frontier are marked out in this way. When the wall came down on November 9, 1989, East and West Germans were eager to get rid of all traces of it.

Following the Berlin Wall Trail toward the center of town, it will lead you to the government district, Spree River and the iconic Brandenburg Gate — which stood in a kind of no man's land when the city was still divided.

Checkpoint Charlie remains the most famous former crossing point between East and West Berlin. Tourists now stop there to snap a picture, under a replica sign reading: "You are leaving the American sector."

More than 300 watchtowers used to overlook the Berlin Wall, allowing East German border guards to catch people trying to flee. Only a few were left standing, such as this concrete structure near Potsdamer Platz, now listed as a historical monument.

Another classic attraction that can't be missed on the Berlin Wall Trail is the East Side Gallery. International artists painted an array of colorful murals on this 1.3-kilometer (0.8-mile) long stretch of the wall in 1990, making it one of the largest open-air galleries in the world. This depiction of Leonid Brezhnev and Erich Honecker kissing is one of its most famous murals.

The route continues through the city proper until it reaches the suburbs and finally Potsdam's Glienicke Bridge, where spies used to be exchanged in Cold War times. In 1962, a KGB agent and an American pilot were swapped at this very place. The story served as inspiration for Stephen Spielberg's 2015 movie "Bridge of Spies."

Large parts of the route pass through forested areas surrounding the city. Right on the bank of the Havel River, this watchtower in Hennigsdorf, about 20 kilometers (12.4 miles) northwest of Berlin, houses a small museum on the history of the frontier. It's free to visit.

Returning to the city, you'll be greeted by a cherry tree avenue in Pankow, which is most spectacular toward the end of April, when the trees are in full bloom. The Japanese donated some 10,000 trees "to bring peace in the hearts of the people." They were planted in different sections of the former wall. This avenue is right by Bösebrücke, the first crossing to open on the day the wall came down.