Joshua 8:29

‘ And the king of Ai he hanged on a tree until the evening, and at the going down of the sun Joshua gave a command, and they took his carcase down from the tree and tossed it down at the entering of the gate of the city, and raised on it a great heap of stones to this day.'

The king of Ai was hanged on a tree. He may well have been killed first, compare Joshua 10:26; Deuteronomy 21:22; 2 Samuel 4:12. This was the token of a criminal and one who was accursed (Deuteronomy 21:22). It demonstrated why Ai had been ‘devoted', because it was a sinful city full of all the abominations of the Canaanites. But the body could not remain there after nightfall lest it bring defilement on the land (Deuteronomy 21:23) and so at the going down of the sun it was taken down and given an ignominious burial. The pile of stones heaped on it were a permanent witness to YHWH's victory and to the end of sinners. Everyone who passed by that heap of ruins would see the pile of stones and would remember what YHWH had done for Israel and what He had done to the king of Ai.