Plinth of the Stalin Statue. That is, everything on the flat space and more you can’t see used to be hundreds of tons of stone statue of Stalin; it was finished shortly after his death and was difficult to remove when the winds changed under Khruschev. Ultimately they had to use dynamite. In Letna Park, where the demonstrations which brought down the regime happened.
John Lennon Wall. The US Ambassador put this under his personal protection in 1989. Peace and freedom-themed graffiti from every part of the world.
Secret police prison. Now a hostel. If you ask for the Presidential Suite you get Vaclav Havel’s 8 x 8 cell.