Achaemenid Settlements > Babylon
In 539 BC the Achaemenid Empire invaded Babylon under the leadership of Cyrus the Great and the city fell during the Battle of Opis that same year. The Persian victory at Babylon was nothing short of military genius. Much like Alexander sieging Tyre by building an elaborate land bridge, Cyrus concocted an elaborate plan to manipulate the environment in order to win the battle.
Since the walls built during Nebuchadnezzar's reign were so impregnable the Persians had to come up with another way to conquer the city. By tasking his engineers to divert the Euphrates river, Cyrus managed to decrease its depth to a wadeable level.
During one of the major religious feasts in Babylonian culture the Persians launched their attack. They simply walked right around the walls into the city and according to some accounts took it without a fight.
However, this was contradicted by reports that there had to be repairs made to walls and some sections of the city so there was most likely some kind of confrontation. Despite being conquered by the Persians, Babylon was not destroyed and instead flourished once more.
Under the Persians Babylon became a center for art, culture and education once more. In fact the Persians held the city in such high regard that Cyrus the Great made it one of the administrative capitals of the empire.
It was rumored that during this period great Western philosophers and mathematicians such as Thales of Miletus and Pythagoras studied at Babylon and developed their theories from there. It was at Babylon that Pythagoras was reported to have developed his theorem of A2 B2 = C2.