Loads of articlse on line - this is a good choice. Reason was the defection of East Germans into the Western sectors. You will appreciate there was a geographical problem. When the former Germany was partitioned (and some land transferred to Poland) Berlin was entirely in the new soviet controlled East Germany. The capital itself was split into western and eastern controlled sectors - so it was relatively easy to get across the city or from the outside - condition being harsh in the East. The wall was the answer - lots of dramtic and daring attempts to tunnel under it and fly over and so on with regrettable loss of life. You also have the political dimension when Kennedy was filmed in front of it making the famous speech that he was a Jam Donut (Ich bin ein Berliner) - presumably he meant he was a citizen of Berlin and shared their plight.