Charles Ellicott Commentary Numbers 31:10

"And all their cities in the places wherein they dwelt, and all their encampments, they burnt with fire." — Numbers 31:10 (ASV)

All their cities wherein they dwelt— Better, all their cities in their dwelling-places. This expression is explained by a reference to Joshua 13:21, from which it appears that the five kings or chiefs of the Midianites who are mentioned in Numbers 5:8 lived in the territory that Sihon, king of the Amorites, had wrested from the Moabites. The Midianites were a nomad people, and were not likely to have built cities for themselves.