Matthew Poole and continuators on Esther 1:2

2 That in those days, when the king Ahasuerus sate on the Throne of his kingdom , which was in Shushan the palace ,

i. e. Either was lately advanced to it: or rather was settled in the peaceable possession of it.

Or, the castle: or the chief or royal city, as both Jewish and Christian Interpreters render it. Shushan might be the proper name of the Palace, which thence was given to the whole City. Here the Kings of Persia used to keep their Courts chiefly in Winter, as ordinarily they were in Ecbatana in Summer.

Marginal references: Or, chief city.