Prior to 1952, the border between East Germany and western occupied zones was easily crossable in almost all places. This changed as the numbers leaving grew: the Soviets suggested instigating a ‘pass’ system to stop free movement between East and West Germany. To make this effective, however, there would have to be something stopping people crossing the border at other places.
Barbed wire fencing was erected across the inner German border, and it was closely guarded. However, the border in Berlin remained open, if slightly more restricted than before, making it by far the easiest option for those who wanted to defect.