All these were cities fortified with high walls, gates, and bars, besides very many unwalled villages. And we devoted them to destruction, as we did to Sihon the king of Heshbon, devoting to destruction every city, men, women, and children.
#deuteronomy 3:5-6

Except for Moses, the Hebrews had no experience in siege warfare when they invaded Bashan. Maybe he taught them in the wilderness or maybe God encouraged the pride of the Amorites so they would leave their fortified cities to fight in the open.