Was it prudent to make a party for residents of Shushan after the party for everyone?
Torah Temimah citing Megilah 12a #1: Yes. First he endeared subjects from far away, for he can always appease those close by.
Torah Temimah citing Megilah 12a #2: No. First he should have endeared subjects of his region, so they would support him if the others rebel.
Malbim: Yes. The final party was for the officers and all Anshei Shushan, to show that "lemi'Gadol v'Ad Katan" (big and small) are the same; they are all his slaves.
Why does it say "l'Chol ha'Am ha'Nimtza'im"?
Malbim: Since Achashverosh came from outside to Shushan and put his throne there, he called to Bnei Shushan who are found there, and made a party in Chatzer ha'Melech. The etiquette of Paras allows only officers to go there. He showed that commoners are no less than officers; all are his slaves 1 . Bnei Shushan, of his capitol, honor is proper for them like for all the officers.
What are "Ginas" and "Bisan"?
Rashi: Ginas is where vegetables grow (a garden). Bisan is planted with trees (an orchard).
What do we learn from "ba'Chatzar Ginas Bisan"?
Torah Temimah citing Megilah 12a #1: Everyone was placed in one of these places, according to his prestige 1 .
Torah Temimah citing Megilah 12a #2: Achashverosh put everyone in the courtyard, but they did not fit 2 . He put them all in the garden, but they did not fit. Finally, he put them in the orchard, and they fit.
Megilah 12a #3: He put them in the courtyard and made two openings; one was to the garden, and one was to the orchard. Vilna Gaon - i.e. Hashem gave man choice to decide to which world he will go (refer to 1:5:4:1*).
Vilna Gaon: The king normally makes a house in his garden, and a Chatzer in front of the garden; it is called Bisan. Even the Chatzer, its floor was of "Bahat va'Shesh?" (6)
Vilna Gaon: There are three worlds. The greatest Tzadikim are in the highest (Bisan). Stam Tzadikim are in the middle (Gan Eden). The lowest (this world) is like a corridor in front of the world to come.