The cities that stood still in their strength - The word תלם tillam, which we translate their strength, and the margin, their heap, has been understood two ways.
2. The cities which were situated upon hills and mountains, which, when taken, might be retained with little difficulty. In this sense the place is understood by the Vulgate, as pointing out the cities quae erant in collibus et tumulis sitae, "which were situated on hills and eminences." As the cities of the plain might be easily attacked and carried, Joshua destroyed them; but as those on mountains, hills, or other eminences, might be retained with little trouble, prudence would dictate their preservation, as places of refuge in any insurrection of the people, or invasion of their adversaries. The passage in Jeremiah, Jeremiah 30:18, Jerusalem shall be builded on her own heap, תלה tillah, if understood as above, conveys an easy and clear sense: Jerusalem shall be re-established on her Own Hill.
Render: “But the cities standing each on its own hill” (compare Jeremiah 30:18). The meaning is simply that, with the exception of Hazor, Joshua did not burn the cities, but left them standing, each on its former site. This site is spoken of as a hill, because such was the ordinary site chosen for cities in Canaan (compare Matthew 5:14).