Joshua 6:20 a ‘So the people shouted when they blew with the trumpets, and so it was that when the people heard the trumpet-sound, the people shouted with a great shout.'
Note the concentration on the noise made. The trumpets sound and the people shout. ‘The trumpet-sound' is literally ‘the sound of the trumpet', the singular drawing attention to the sound rather than the trumpets. This was the long blast of Joshua 6:5. Now the city would recognise that the moment had come for them to put up stout defence. But they did not realise what was about to happen.
Joshua 6:20 b ‘And the wall fell down flat, so that the people went up into the city, every man straight before him, and they took the city.'
What caused the wall to fall flat? The basic answer was, YHWH. Whether it was by an earthquake or tremor, or by resonance from the noise made which reacted on unstable walls possibly crowded with defenders, it was to be seen as at the instigation of YHWH. Thus it was not a matter of forcing their way through a breach in the walls but simply one of going straight forward and clambering over the fallen stones. The relatively few defenders, numbering in hundreds (even though crowded with people from the surrounding countryside), and numbed by what had happened, had no chance against the much larger Israelite force, numbering probably around six hundred military units (Exodus 12:37).