“and the slopes.” Joshua 10:40 is a brief summary of the areas where the majority of the Canaanites lived and where Joshua conquered. The summary is from the hill country west, and omits the Arabah, but the only major city omitted by that is Jericho. “And the slopes” refers to the area between the “hill country,” which is much more mountainous, down into the shephelah, which is much flatter but not as flat as the coastal plain.
“the Shephelah.” The geography of Israel, for most of its length, is divided into four geographical zones: the coastal plain by the Mediterranean Sea; the Shephelah, which is the area of rolling hills east of the coastal plain and between the coastal plain and the hill country; the hill country, which is the inner country that is quite mountainous; and the Jordan Valley, much of which is very arid and referred to as the Arabah.
“devoted to destruction all that breathed.” In obedience to Deuteronomy 20:16.