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).

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.
1. As signifying those cities which had made peace with the Israelites, when conditions of peace were offered according to the command of the law; and consequently were not destroyed. Such as the cities of the Hivites; see Joshua 11:19.
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.

But [as for] the cities that stood still in their (g) strength, Israel burned none of them, save Hazor only; [that] did Joshua burn.
(g) Which were strong by situation and not hurt by war.

But as for the cities that stood still in their strength,.... Whose walls were not demolished when taken, as Kimchi and Jarchi interpret it, or that "stood upon their heaps" (y); upon an eminence, being built on hills and mountains:
Israel burned none of them; but reserved them for their own habitations, being well fortified, and having no need of new walls being built to them, or being in a very agreeable situation:
save Hazor only, that did Joshua burn; because it was the chief city where the scheme was formed, and the combination against Israel was made, and was the rendezvous of the confederate forces against them: the Jews have a tradition (z), that God said to Moses, and Moses said to Joshua, that he should burn it, and that only.
(y) "super tumulum eorum", Montanus; "quae erant in collibus et in tamulis sitae", V. L. (z) Bereshit Rabba, sect. 81. fol. 71. 1.

as for the cities that stood still in their strength--literally, "on their heaps." It was a Phœnician custom to build cities on heights, natural or artificial [HENGSTENBERG].

In their strength - Hebrews. with their fence, walls or bulwarks, that is, which were not ruined with their walls in taking them. Save Hazor - Because this city began the war, and being the chief and royal city, might renew the war. If the Canaanites should ever seize upon it: which in fact they did, and settled there, under a king of the same name, Judges 4:2.