Hi Bob,
Oddly enough, it seems that the B entrance (Second Entrance as it was called then) was the Private Entrance reserved for members of the Imperial Family and that most visitors who called on the Emperor used the A (First) Entrance. This would make some sense though, as the Waiting Room and the Tsar’s Study were very close to this entrance.
But the Main Visitors’ Entrance was the C (Third) Entrance and the dignitaries who were received in the formal Reception Rooms used this entrance. (The D Entrance was used by members of the Household). As you will understand by reading the sometimes misleading translation of the Court Journal descriptions of Diplomatic receptions in the « American Ambassadorial Palace Visits » on the Main Palace Site (
http://www.alexanderpalace.org/palace/avisit.html ) , most of the time the dignitaries arrived in Court carriages to the 3rd Entrance and were met there by Court officials who led them in procession, usually through the Reception Room No. 21, the Crimson Sitting Room (20), the White Hall (19), then through the Parade Halls (12, 11, 10), the Mountain Hall (9), the Great Library (3) a Small Library (2, also called Small Sitting Room (2) into the Formal Corner Reception Room (1, « Corner Sitting Room »). Interestingly enough, it was in this room that the Emperor received Ambassador’s Letters of Credence. Not very formal don't you think?! I would have done it in a Throne Room!!