I've only been to one concentration camp (not sure there's a need to visit more than one), Majdanek, a relatively small one but the jarring thing is it's not out in the middle of a forest but right on the edge of town. In fact, I remember going to look at a flat to rent and the owner felt obliged to point out that you could see the concentration camp from the windows, calling it "a rather sad view". But in terms of the buildings there, the ones that most left an impression on me were the ovens, just two of them, but with a dissection table and a sluice in the middle of the room, and a room full of shoes taken from victims. (Who made those ovens? Can you imagine the ordering process? "Is that Schmidt's bakery equipment Gmbh? We want two ovens, about 2m long and 1m wide.")