The drawing may illustrate where the gate was relative to the north wall. However the drawing is not accurate. The city of Babylon was built on both sides of the Euphrates river with walls on both sides of the river and with streets that were perpendicular to the river. There were hugh brass gates through which inhabitants could carry water from the river. These gates could easily be shut to keep out would be conquerers but were carelessly left open during Cyrus's assault on the city and therefore easy access was made in the conquest of the city in 606 BCE. The gates are mentioned in Isaiah's prophecy of Cyrus conquest. The historical description and the gates were recorded by Herodotus a fourth century BCE Historian. See his description here in Book 1:179.
Isaiah makes an incredibly accurate prediction of how Cyrus entered the city and makes particular mention of the gates of Brass. See it here