Jamieson, Fausset and Brown on Jeremiah 51:31

31. (See on Jr 50:24).

One post--One courier after another shall announce the capture of the city. The couriers despatched from the walls, where Cyrus enters, shall "meet" those sent by the king. Their confused running to and fro would result from the sudden panic at the entrance of Cyrus into the city, which he had so long besieged ineffectually; the Babylonians had laughed at his attempts and were feasting at the time without fear.

taken at one end--which was not known for a long time to the king and his courtiers feasting in the middle of the city; so great was its extent that, when the city was already three days in the enemy's hands, the fact was not known in some parts of the city [Aristotle, Politics, 3.2].