Aquatic?

[Tuna]

Gandalf Parker said:
Why would a land nation want a fort underwater?

Caelum that builds an underwater fort and relocates all his mages there gets full protection vs all overland spells quite cheaply. Very efficient in the very late game, as you can place all your clams there. Should someone try sieging it you can always break the siege with some mages/chaff.

[Sheap]

This tactic is of course not only available to Caelum, Ermor and Pythium can do it just as well (Ermor even better, because their troops are almost all amphibious, and they are cold immune like Caelum). But in reality dome spells are a better defense than being in the water.