Question

why was the berlin wall built?

Answer 1

After WWII Germany was divided into 4 zones (British, US, France and the USSR). Berlin was also divided into 4 zones. Least to say...this was a pretty big problem for Russia, noting that Berlin was in the USSR. BUT still divided you see.

It separated Germany, and the citizens from the communist zone and the Western area. Another reason for the wall was to protect the style of economy, that East Germany and the USSR had.

Without the wall, goods were flooding into the East German zone, and the people living in the East were working in the West and bringing Western currency back to the East side.
THIS would mess things up as you can guess, but they had an idea :)

To control the influx of goods, currency, etc. - to minimize the influence of the West Berlin economy in the East, the wall was built in 1961.

Even though the wall long since has lost the purpose it was built for, you can still visit parts of it.