[Ertial]

Classes shouldn't be realm specific. You have spies who would steal that and the human mind couldn't bear that other people could do things they can't, and after the first fight the would copy the tactics and way of fights of the others.

All realm should have something specific. Maybe the nature likes Syrtis more, the Mountains(yes, I know, that is nature too, but for Syrtis I meant forests and grassland) like Alsius and sandstorms like Ignis, so they won't hurt them(very much).
Syrtis wants to defend themselves, so they have protection domes(?) around the saves/gates. Maybe they installed a spell just before the wall, which makes the land behind it look like a thick forest without path, but Syrtis people can walk trough it.
Ignis has traps for Syrtis installed and is very mean to them(and trees). Alsius just wants more terrain, so they won't make alliances, unless it's good for them. Realms can speak to each other, except Ignis-Syrtis-Ignis, they hate each other so much the don't learn each others language and don't want to understand each other.

EDIT: Found the name of the game, University at War: Earth Assault