John Gill Commentary Deuteronomy 3:5

"All these were cities fortified with high walls, gates, and bars; besides the unwalled towns a great many." — Deuteronomy 3:5 (ASV)

All these cities were fenced with high walls, gates, and bars ,
&c.] That is, all the cities in the kingdom of Bashan; and though they were, it hindered not their falling into the hands of the Israelites; and this might serve to encourage them against those fears they were possessed of by the spies, with respect to the cities in the land of Canaan; see (Numbers 13:28) (Deuteronomy 1:28)

besides unwalled towns a great many ;
small towns and villages adjacent to the several cities, as is common.