Stairway
From The Big Bang Theory Wiki
Because the elevator is broken, the stairway is the only way to get to the upper floors of the building. Conversations which move the story ahead often take place during treks up and down these stairs (e.g. 0105).
When Leonard and Leslie were having a date in the apartment, Sheldon was relegated to the stairway to play Super Mario on his laptop, plugged in with a long extension cord (0202).
