John Gill 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)

And they burnt all their cities wherein they dwelt
} So far as they went into the country, which, perhaps, might not be the whole land of Midian:

and all their goodly castles with fire ;
which were either the palaces of their princes and nobles, or such like great personages; or they were fortified places, for the security of their cities and country: the Targums of Onkelos and Jonathan interpret them of their houses of worship, their temples, and their altars.