What is the meaning of "Pose'ach Es Kesef Mo'av"?
Rashi: I will open the fortresses of Eretz Mo'av on the border. Kesef is the end of the border, like "vela'Kasef ha'Shenis" (Shemos 27:16). The Targum of "El ha'Kasef" (ibid. 38:14) is l'Ivra. Radak - Mo'av trusted in their fortified cities. I will open them, so the enemy will enter them!
Malbim: The enemy will not enter from the side of the Midbar, rather, from the side where there are cities. Its Churban will begin from there.
Why does it say "me'Arav Miktehu"?
Radak: This is a repetition - from the cities at the end of its border. One fortifies them more than other cities.
What do we learn from "Tzvi Eretz Beis ha'Yshimos"?
Radak: It is the most coveted of its land - Beis ha'Yshimos.
