Designs to Keep People Out or Keep People In

I loved this old city defense wall in the heart of downtown Krakow. It's hundreds of years old and of course, was built to try to keep out any number of attacking armies that frequented Krakow over the centuries.
Then there is the barrier built to keep people inside at Auschwitz which is less than an hour's drive from the old city wall. Something of a study in constrasts, isn't it?