Though much of Birkenau was destroyed by the retreating Germans, the size of the place, fenced off with long lines of barbed wire and watchtowers stretching almost as far as your eye can see, will give you some idea of the scale of the crime; climb the tower at the entrance gate to get the full effect. If you’re not part of a tour, make sure to leave enough time (at least an hour) to walk around the camp.
It was actually at Birkenau, not Auschwitz, that most of the killing took place. Massive (175 hectares) and purpose-built for efficiency, the camp had more than 300 prison barracks – they were actually stables built for horses, but housed 300 people each. Birkenau had four huge gas chambers, complete with crematoria. Each could asphyxiate 2000 people at one time, and were fitted with electric lifts to raise the bodies to the ovens more quickly and conveniently.