The double entrance stairs onto the terrace from the west are a prelude to the Porte des Nations (Gate of All Nations).
The original entrance, however, was to the south, to provide more direct access to the Palace of Darius and probably the Treasury. It is also on this facade that the large trilingual inscription of Darius was located (link)
The gate of all nations was built by Xerxes.
Description
Function of this door. Despite its astonishing angle, the various “ambassadors” of the nations passed through this gate before heading towards the Apadan to deliver gifts for the Great King.
Origin: first idea, in Pasargadae (palace R), then in Susa, the “gate of Darius”.
Darius is the successor of Cambyses after adventures recounted on the Bisutun inscription
