During the Second Temple period, there were five gates leading to the Temple Mount, two on the Southern Wall and one each on the Western, Eastern and Northern walls. The Mishna in Massekhet Middot (1:3) teaches that the gate on the Eastern side was called “The gate of Shushan” and it was decorated with an engraving of the Persian capital, Shushan.
Two opinions are offered by the Talmudic Sages to explain why Shushan appeared there. Rav Ḥisda and Rav Yitzḥak bar Avdimi weighed in on this question. According to one it was so that the people would know from whence they came; according to the other it was so that the fear of the ruling government should be upon the people.
According to Rabbinic tradition, the Second Temple period began with the proclamation made by King Cyrus of Persia who permitted a return of Jews to the Land of Israel and the rebuilding of the Temple (see Sefer Ezra 1:1-11). Even after the rebuilding of the Temple, the Jewish community in Israel remained part of the Persian Empire, until its capture by Alexander the Great, an event that began the era of Greek rule.
With this in mind, Rashi explains the first suggestion as follows: The people were reminded that they were given permission to return to Israel by the Persian government, and that they should give thanks to that government for allowing them to do so. Rabbeinu Gershom suggests that the people were expected to give thanks to God for the miracle that he performed in directing history so that King Cyrus permitted a return to Israel and a rebuilding of the Temple. According to Rabbeinu Ḥananel the point was to get the people to remember the sins of the previous generations that led to their exile to Babylonia, so that they would share this history with their children.