“the cities that stood on their mounds.” This refers to the cities that were built on (“stood on”) the ruins, the mounds, of the previous cities. So by Joshua’s time the Canaanites had fought each other and destroyed and rebuilt their cities, building cities on top of the ruins of the city underneath. The reason for building one city on top of another in the same spot was that to be a successful and prosperous city, it needed defensibility, water, and being near a trade route. If a city had those three things, the location was favorable and cities would be built on top of one another. Many of the tels in Israel were well along by the Middle Bronze Age.
“Israel burned none of them, except Hazor only.” Jericho, Ai, and Hazor, were the only three cities that Israel burned in the conquest of the Promised Land, but Joshua 11 is about the northern conquest and Hazor was the only northern city that Israel burned so Hazor is the only city mentioned in this context to have been burned. Israel was to live in cities and houses that they did not build (Deut. 6:10-12), so in general, they did not destroy the buildings of the towns they conquered. This challenges archaeologists to be careful in trying to find evidence for the Israelite conquest of Canaan because the burn or destruction layer that is often found when one culture conquers another is often not present in the Israelite conquest of Canaan.