Adam Clarke on Joshua 6:5

The wall of the city shall fall down flat - Several commentators, both Jews and Christians, have supposed that the ground under the foundation of the walls opened, and the wall sunk into the chasm, so that there remained nothing but plain ground for the Israelites to walk over. Of this the text says nothing: - ונפלה חומת העיר תחתיה venaphelah chomath hair tachteyha, literally translated, is, The wall of the city shall fall down Under Itself; which appears to mean no more than, The wall shall fall down From Its Very Foundations. And this probably was the case in every part, though large breaches in different places might be amply sufficient to admit the armed men first, after whom the whole host might enter, in order to destroy the city.