This new one per faction/player building will grant you some interesting gameplay features:
- A real feeling of having a capital city, ever found your starting city to be underdeveloped and meaningless? This is for you
- Fully implemented AI priority to use it, this will be the first thing they build, so you will on equal grounds with them or you can handicap yourself for a more defying game. I noticed the AI, would only be interesting to play against, if you waited for them to build their forces. Hopefully this helps them to grow faster and enrich your game experience. For the record the AI will even build these in campaign in v1.08, except for Pariden for some reason 
- The ability to get a sentinel units, which can't move around, to defend your initial city. The AI will also make them soon as they come free, unfortunately sometimes, they move them out of city.
- A cheap big building to make it easier to build if you start in a location surrounded by resources
- A mighty flashy giant tower to feed your sovereign's ego 
SCREENSHOTS
EMPIRE Wizard Fortress

Actually I commented the lightning effect out of the final release, as it was overloading the tile too much, you can easily re-enable it if you like it
Empire fortress, final version:

Kingdom version of the wizard fortress, this was my first building in the tile editor (then it had no effects, though)

Changelog v2.0:
Introduces magic spirit unit
Enables ArcaneMagic resource and uses it for maintenance
Empire Design reworked to be more similar to the MoM wizard fortress, it's not only different colours now.
TO DO:
Give the guardian spirit, the MoM ability to increase defence of all units on your side by one [If you know how to make global affecting spells in tactical combat, lend me a hand
]
Fix the AI to not move Guardian Spirits out of city
Rework the guardian spirit model.
Learn how to cap ArcaneMagic production, so you can only produce one guardian spirit and avoid exploits
DOWNLOAD:
http://dl.dropbox.com/u/2613529/Elemental/WizardFortress.zip