When he sat on the throne of his kingdom, the city Susan was the capital of his kingdom.

in those days, when the king Ahasuerus sat on the throne of his kingdom, which was in Shushan the fortress,

in those days, at the sitting of the king Ahasuerus on the throne of his kingdom, that is in Shushan the palace,

That in those days, when King Ahasuerus was ruling in Shushan, his strong town,

that in those days, when the king Ahasuerus sat on the throne of his kingdom, which was in Shushan the castle,

Sat on the throne of his kingdom - Having subdued all his enemies, and brought universal peace to his empire. See the commencement of the introduction.
Shushan the palace - The ancient city of Susa, now called Shuster by the Persians. This, with Ecbatana and Babylon, was a residence of the Persian kings. The word הבירה habbirah, which we render the palace, should be rendered the city, εν Σουσοις τῃ πολει, as in the Septuagint.

[That] in those days, when the king Ahasuerus (c) sat on the throne of his kingdom, which [was] in Shushan the palace,
(c) That is, had rest and quietness.

That in those days, when the King Ahasuerus sat on the throne of his kingdom,.... Not only was placed upon it, but settled in it; after Xerxes had subdued Egypt, in the second year of his reign (e), and enjoyed great peace and tranquillity:
which was in Shushan the palace; that is, the throne of his kingdom was in Shushan, the royal city of the kings of Persia; of which see Gill on Nehemiah 1:1, Daniel 8:2.
(e) Herodot. ib. (Thalia, sive, l. 3.) c. 7.

The words: in those days, take up the chronological statement of Esther 1:1, and add thereto the new particular: when King Ahashverosh sat on the throne of his kingdom in the citadel of Susa. שׁבת does not involve the notion of quiet and peaceable possession after the termination of wars (Clericus, Rambach), but that of being seated on the throne with royal authority. Thus the Persian kings are always represented upon a raised seat or throne, even on journeys and in battle. According to Herod. vii. 102, Xerxes watched the battle of Thermopylae sitting upon his throne. And Plutarch (Themistocl. c. 13) says the same of the battle of Salamis. Further examples are given by Baumg. l.c. p. 85f. On the citadel of Susa, see Nehemiah 1:1, and remarks on Daniel 8:2.

Sat - Was settled in the peaceable possession of it. Shushan - The chief or royal city. 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 in winter, as at Exbatana in summer.