Charles Ellicott Commentary Esther 1:2

"that in those days, when the king Ahasuerus sat on the throne of his kingdom, which was in Shushan the palace," — Esther 1:2 (ASV)

Shushan. —Susa. Mentioned also in Nehemiah 1:1. It was the usual residence of the Persian kings. (See Herodotus 7.6.)