Adam Clarke on Joshua 8:29

The king of Ai he hanged on a tree - He had gone out at the head of his men, and had been taken prisoner, Jos 8:23; and the battle being over, he was ordered to be hanged, probably after having been strangled, or in some way deprived of life, as in the case mentioned Jos 10:26, for in those times it was not customary to hang people alive.

As soon as the sun was down - It was not lawful to let the bodies remain all night upon the tree. See the note on Deu 21:23. The Septuagint say the king of Ai was hanged επι ξυλον διδυμον, upon a double tree, which probably means a forked tree, or something in the form of a cross. The tree on which criminals were hanged among the Romans was called arbor infelix, and lignum infelix, the unfortunate, ill-fated, or accursed tree.

Raise thereon a great heap of stones - This was a common custom through all antiquity in every country, as we have already seen in the case of Achan, Jos 7:20.