well The Berlin Wall was build over a very long period and there were different steps of building up the wall.
On the first day (August 13, 1961) the border in Berlin was completly closed and in generally it was not possible for East Germans to set a food to the other part of the city.
East German border guard patrolled on the whole border through Berlin. At first a barbed wire was used but on August 15, 1961 the first Wall was built up.
Until the fall of the Berlin Wall in 1989 the Wall system was extended permanently.