The design of the iconostasis is what iconostases looked like in the early Church. Now, not every church had a such a large three-sided iconostasis protruding into the nave, but the iconostasis had a half wall surmounted by pillars then a horizontal beam. The openings are covered by curtains and we can already see small icons of the Mother of God and Christ the Teacher on either side of the Royal Door (the central door). Over time, the icons in iconostases grew to fill the whole opening. The three doors grew to fill their openings and gained icons. The Royal Door gained an icon of the Annunciation (the Mother of God on one door and St. Gabriel on the other) and often times the Four Evangelists as well. The north and south deacon doors gained icons of either deacon saints or angels. The other openings gained icons as well. North of the north door is usually St. Nicholas the Wonder-worker, Archbishop of Myra, and south of the south door is the patron of the church. The icons of the Mother of God and Christ the Teacher go between the doors. The iconostasis came to be surmounted by icons of the 12 apostles, although some churches will instead put them on high on the apse wall, forming an icon of the Mystical Supper (Last Supper).
The West however, modified the original design in another direction. Instead of filling the openings with icons, the curtain and small icons were removed and the crucifixion scene was added to the top above the central door, which came to be the only door. The Icon Screen came to be named the Rood Screen, with "rood" being a Middle English word for "cross". Eventually, the whole screen was dropped down to make the altar rail of today.
Here's the outside of the Hagia Sophia: