On June 10, 1940, the Gestapo took control of Terezìn (Theresienstadt), a fortress, built in 1780-1790 in what is now the Czech Republic, and set up prison in the Small Fortress (Kleine Festung). By 24 November 1941, the Main Fortress (grosse Festung, ie the town Theresienstadt) was turned into a walled ghetto. The function of Theresienstadt was to provide a front for the extermination operation of Jews. To the outside it was presented by the Nazis as a model Jewish settlement, but in reality it was a concentration camp.
Theresienstadt was also used as a transit camp for European Jews en route to Auschwitz and other extermination camps. Dr. Siegfried Seidl, an SS colonel, served as the first camp commandant in 1941. Seidl oversaw the labour of 342 young men, known as the Aufbaukommando, who converted the fortress into a concentration camp. Although the Aufbaukommando were promised that they and their families would be spared transport, eventually all were transported to Auschwitz-Birkenau in 1943, for Sonderbehandlung, or gassing without selection.
From the guide to the Theresienstadt Activity Report (microfilm), 1942, (Wiener Library)