Well here is what I learn after they came back from their time in Terezin and also by researching the subject.
Terezin was constructed as a Fortress in the 1780/90 time. It was never fully finished, and in 1940 the Gestapo took over and set up a prison for "Priviledge Jews".
What is being a "Priviledge jews" you may ask, well apparently it was for people who had some kind of education(Artist, Writers, Scientist, Jurists, Diplomats, Musicians...etc...). It was used by the Nazi on their Propaganda and was (probably the only) the camp that was mostly visited by Red Cross. The Barracks of the Fortress were build to house 7000 individual. During the "Working Camp" in 1940-1945 it accomodated over 50000. Some survivors claimed it reached 75000.
Terezin camp was a source of free jewish labor (slave) for the Germans. Prisoners were assigned different tasks: Mining mica, manufaturing boxes or coffins, spraying military uniforms with white dye. According to ex-prisoners, it was also a sorting re-distribution center for clothing items confiscated from Jews. The baggages were taken away and send to Terezin where it would have been sorted out and send out all over Germany for the people who were bombed out.
Married couple were given a single room and were able to stay together in their sleeping quarter. Because Terezin was design to house "Priviledge" Jews, it did have the cultural life that most camp didn't. Jazz ensembles, Chamber Orchestra Groups, education for children etc...They also overcame a lack of water by building water pipe system and water supply system.