"Shudad" is masculine, and "Areha" is feminine!
Rashi: Verses often switch between masculine and feminine. The masculine refers to the nation, and the feminine to ha'Kenasah.
What is the meaning of "v'Areha Alah"?
Radak: Its [multitude of] cities ceased. The same applies to "Yetzu'i Alah" (Bereishis 49:4), "ka'Alos Gadish" (Iyov 5:26).
Radak citing Targum Yonasan: Their cities became desolate.
Radak citing his father: [The plunderer] came upon their cities.
Malbim: The city dwellers ascended the stronghold [and the Giborim descended to be slaughtered. This is opposite the norm, in which the Giborim ascend on the wall to fight, and the [other] city dwellers descend and hide. We see that they are not Anshei Chayil Milchamah! Since Mo'av was plundered, this shows that their 'Giborim' are not strong!]
