Umm... no, not the Dunmer fortresses; I meant the Dwemer Citadels at Red Mountain, Kagrenac's Study & Library and such, since taken over and renamed after Dagoth Ur's Ash-vampires.
If these fortresses are the Shield, they are shielding something, alright, Red Mountain, which according to some serves the same function as the Adamantine Tower and the Whitegold Tower. The volcano Red Mountain was previously named Vvardenfell, see PGE 1, and about any older map.
But why would the Dunmer keep the name Vvardenfell? The Tribunal had to go to Red Mountain annually to renew their bond with the Heart of Lorkhan. Maybe it's precisely because the three owed their powers and God-hood to Dwemer tools and studies, to harness the power of the heart, that they didn't dare change the name. I mean Almalexia had no trouble renaming Mournhold to her name. Maybe the Dwemer hate was reserved for the masses, propaganda, so-to-speak, Sotha Sil certainly was obsessed with everything Dwemer. Speculating here