The most common forts are usually Watchtower (cheap, can be put everywhere, costs nothing), and the Castle (costs points, more expensive to build, taker a longer while). The Watchtower is especially nice for a nation like Caelum, as you can recruit all your mages everywhere, unlike Man, whose mages are only available in your capital.
Dominion strength is also helpful for some special effects, like the automatic generation of troops (Ashen Empire and Soul Gate for Ermor, Carrion Woods for Pangaea), killing off your own population (same themes), or getting an income bonus (C'tis Miasma, where you get +1% income per level of Dominion in all provinces). And of course, if you lose all your Dominion, you are dead.
