What famously divided Berlin during the Cold War?

The Berlin Wall was a guarded concrete barrier that encircled West Berlin from 1961 to 1989, separating it from East Berlin and the German Democratic Republic (GDR; East Germany). Construction of the Berlin Wall was commenced by the government of the GDR on 13 August 1961. It included guard towers placed along large concrete walls, accompanied by a wide area (later known as the "death strip") that contained anti-vehicle trenches, beds of nails and other defenses. The primary intention for the Wall's construction was to prevent East German citizens from fleeing to the West.

Source: Wikipedia