Museum of Martyrdom “Under the Clock” has a large collection of correspondence of the political prisoners of the Lublin Castle (letters, secret letters, postcards), prison at Uniwersytecka Street and concentration camps they were sent to (KL Auschwitz, Ravensbrück, Dachau, Sachsenhausen, Lublin, Flossenbürg). In the museum one can see announcements and German orders from 1939–1944.

A separate group is formed of documents and personal mementos after former prisoners of the Castle: ID cards, school certificates, scouting crosses, school shields, family photographs, notebooks and textbooks, objects made in a prison in the Castle.

A large collection is made of documents and personal mementos of prisoners of the German concentration camp in Ravensbrück. The most valuable group of documents constitute secret letters written in urine with information about the conditions in which prisoners were found, and their daily struggle for survival in the camp.

The Museum has a rich iconographic material of Lublin from 1939–1944, as well as photographic archive of former prisoners of the Castle from 1939–1954.
The museum collections include material documenting the activities of the Club/Association of Former Political Prisoners of the Castle and “Under the Clock” (21 volumes of chronicles from 1969–2000).

In recent years, the Museum has obtained valuable artefacts related to Mieczysław Makary Smorawiński – General of the Polish Army during the Second Polish Republic, the Commander of the Corps No II in Lublin, murdered in 1940 in Katyń.