Thanks for the history, and the memories Paul. when I was in Berlin in '66 it was because my dad, then a sergeant in the British Army, was a contestant in a big weightlifting tournament being held in East Berlin. I have memories of ominous looking guards inspecting our car as we drove from West Germany into the East to Berlin, of women with pickaxes and apron covered long skirts doing road works on the autobahn, and going through Check Point Charlie in a military car with my mother. My dad was in our car, in full uniform in front of us. I'm not sure of all the details now, but it seems the guard wanted to stop him - at gun point. When he saw Dad's uniform and the ID he flashed at the window, he backed down, and waved us all through. A scary moment. I think there was some rule that they could not stop military personnel, not sure of the details. I was 11 yrs old at the time. Goodness, and now Checkpoint Charlie is a museum!! I feel old! 
There used to be a platform you could climb onto to look over the wall at Potzdamer Platz. On the other side was yards of tank traps and barbed wire, and East Berlin looked grey and dark, even in daylight. Wooden towers with armed guards watched your every move.