Babylon
In 539 BCE, the Neo-Babylonian Empire fell to Cyrus the Great, the king of Persia, in the Battle of Opis. The walls of Babylon were very high and very thick. The only way into the city was through one of its many gates.  The Euphrates flowed next to the  walls and Cyrus decided to use the river to get into the city. Cyrus' troops diverted the Euphrates river. This caused the level of the river to drop allowing soldiers to enter the city.