I'm thinking about a serie of spell that could boost your fortress/wizard tower. One spell on each circle that gives different bonus. Here are a few ideas of possible bonus :

Air : Flying Fortress, allows to relocate your fortress once per turn, with all associated effects (if, like in MoM, spell casting in battle close to the fortress is easier). Could have restrictions in how far it can move, or building/city size prerequisite.

Earth : Fortress of Abundance : bonus to production, food and/or gold income in the capital city (the one where the fortress is).

Fire : Exalted Fortress : in all combats near your fortress, your unit receive a damage/to hit bonus.

Water : Domed Fortress : your capital city is protected by a dome of water, depleting non-amphibious ennemy units of all movement points when they come close to the capital city (meaning they have to stay a full turn located just near the capital before they can attack).

Life : Healing Fortress : all units around your fortress heal quickly (completely healed at the beginning of each overland map turn ? small regeneration in all combat round ?)

Death : Fortress of Decay : all living (non-mechanical, non-undead) ennemy units near the fortress take damage every overland map turn, or when they first come near it.

That's my first ideas for elemental circles, if people think the idea is good I'll try to propose effect circles too, if not, well, too bad