The border between West and East Germany was already heavily fortified in 1952. So, during the nine years until 1961, West Berlin was indeed a kind of porthole

The border between West and East Germany was already heavily fortified in 1952. So, during the nine years until 1961, West Berlin was indeed a kind of porthole to the west. The reasons why the East German government did not close the border within Berlin are complex. For example the Sowjets first had the goal to make Berlin an independent but unified city. But there were also economical and logistical issues: The border would cut important railway transport routes for East Berlin, for which a replacement was only completed in 1961.