Rashi: You will not need fortified cities. Radak ? a wall surrounds them. They will not need a wall, for they will have Shalom; no one will quarrel with them. It is as if I cut off the wall of your cities. They will not be called Arim, rather, Perazos (open cities) ? "Perazos Teshev Yerushalayim" (Zecharyah 2:8). The same applies to "v'Harasti Kol Mivtzarecha." Dwelling in Perazos is healthier than dwelling in walled cities. Chazal said (Kesuvos 110b) that one cannot force his wife to move from a city to a big city, for dwelling in a large city is hard - "They blessed all the people who volunteered to live in Yerushalayim" (Nechemyah 11:2). This is because there is little air in big cities; open cities are expansive and healthy.

Radak citing Targum Yonasan: I will remove the cities of nations from your land, and destroy their great cities.

Malbim: These are cities that belong to the land and country. Cities of chariots and weapons do not belong to the people, rather, the country; they are called Arei ha'Aretz.