On both sides of the entrance to the Kodesh there were small openings called pishpeshim. The southern opening was always closed and no one ever entered through it, as the verse states: "This gate will be closed... and no man will enter in by it, because Hashem the G-d of Israel has entered through it." (Ezekiel 44:2) The northern opening was opened each morning using two keys. Through it one could enter the compartment and from there to the Kodesh. It was necessary to first enter the Kodesh through the compartment since the main doors of the Kodesh could only be unlocked from inside. (Sources: Tamid 3:7; Midot 4:2). 
Holiness
Contents:   Part I,    Part II,    Part III,    Part IV,    Part V,    Part VI,    Part VII