What was unusual about the conquest of babylon?

Babylon was conquered numerous times over it 2,000 year history. The first time was when Babylon was not much more than a village and the Amorites, under Hammurapi's ancestor Sumuabum, took over at the start of the 19th century BC. The second time was in the mid-16th century BC when the Hittites sacked the city and the Kassite peoples subsequently established a ruling dynasty. Third was when the Elamites briefly took the city c. 1160 BC; followed by the reclaiming of the city's rule within the sphere of the Sumerian city of Isin and kingdom of Sealand in Chaldaea. The fourth conquest was by the Assyrians c. 810 BC when they captured the city by diverting the Euphrates to flood the city. This same flooding technique was used by the Assyrians to reconquer the city after a brief rebellion c. 690 BC. In 612 BC a Chaldaean named Nabopolassar finally overthrew the Assyrian dominance and in fact ended the Assyrian Empire. The sixth conquest of Babylon by the Persians in 539 BC was uniquely accomplished by draining the Euphrates and marching the Persian forces up the dried up river bed to the unfortified banks where the river course ran right through the heart of the city. The last major conquest of Babylon was by Alexander the Great when the city surrendered to him in October of 331 BC after he had defeated the Persian army in a previous decisive battle in the plains of norther Mesopotamia at Gaugamela.