The D Deck Door located on the starboard side of Titanic, served as the front  entrance for First Class passengers. The White Star Line made sure that this space was of the finest design and calculated to impress. Through this door, the first class passengers would enter through one of the two gangway doors on either side into a large entrance foyer. The inside of each gangway entrance was fitted with a pair of decorative wrought-iron gates that hid the steel doors from view inside Titanic

First Class passengers would pass the grand staircase and be welcomed by the stewards who would escort them to their rooms. 

Or, they proceeded to either the dining room or the boat decks to wave farewell to the spectators on the docks. The D Deck Door was open the night when the ship sank. The crew thought they could disembark passengers through the door, but apparently that was not the case. The door was left open during that frightful night. The video below shows a brick model of Titanic and where the D Deck Door was located.