The North and South (liturgically, since we’re not arranged facing true East due to limitations of the space) doors, more commonly called the Deacon’s Doors are on either side of the Royal Doors (sometimes called the Holy Doors - more on them next week), situated in the Iconostasis. The Deacon’s Doors are so named because they are where the deacon (if we had one) would enter and exit, because only priests and hierarchs enter through the Royoal Doors. Generally the “North” Deacon’s Door is the exit from the Altar - where we come out for the Little and Great Entrances, and the “South” is where we enter the Altar.
When do we enter the Altar? Only when we have a reason to, and only if blessed to do so; while our Altar isn’t consecrated and hence anyone can technically enter it, we still hold the space as Holy, so it should not be entered “just because.” On the Deacon’s Doors we find depictions of Archangels Gabriel and Michael, the Messenger and Defender respectively, announcing our entrance to Heaven in the Incarnation and guarding the door thereto. At St. Andrew’s we have Sts. John Chrysostom and Basil the Great above our Deacons’ Doors, because they are the authors of the Divine Liturgies we use most frequently (St. James’ icon is over by the trapeza).