Albert Barnes Commentary Esther 9:6

"And in Shushan the palace the Jews slew and destroyed five hundred men." — Esther 9:6 (ASV)

“Shushan the palace (or the fort)” probably refers to the entire upper town, which occupied an area of more than 100 acres and contained many residences besides the actual palace. The Jews would not have ventured to shed blood within the palace precincts.