1)
What are "Armenoseha"?
1.
Radak: Castles of other cities are built strong and tall, to take refuge there. Those of Yerushalayim, their strength is that Elokim is known in them. He is the refuge of all who seek to take refuge there - "Migdal Oz Shem Hashem Bo Yarutz Tzadik v'Nisgav."
2)
Why does it say "Noda l'Misgav"?
1.
Rashi: When He will dwell there in the future, they will say so.
2.
Malbim: Via this, Hashem is known to be a refuge. The primary strength [of Yerushalayim] is that Hashem dwells in its castles. This is "u'Mehulal Me'od" (verse 2) - they praise Him for He put His Shechinah in Tziyon, and He is its refuge.
