In 1961, Khrushchev had threatened West Berlin, but when President Kennedy had refused to get rid of the troops, the Soviet forces decided to construct a wall separating East and West Berlin, which was know as the Berlin wall. If you want to learn more click Here to watch a short video or read more about why the Berlin wall was built.
After the Berlin wall had been mostly put up, the constructors had to reinforce the wall with bricks and more watchtowers to prevent "prisoners" from escaping. Click Here to learn more about reinforcement of the Berlin wall.
The Berlin wall was a 9 foot tall wall that stretched across Berlin with barbed wires above the wall to be certain no one could climb over.