There is a very rare type of Indy's comprised of Unicorns, Pixies and Leprechauns, which can appear and attack regular Indy provinces, capture them, and then start building magical fantasy cities (not just forts!!). And if you capture these cities (though be warned, Pixies are tough as nuts!), you will find they contain things like libraries, factories, stables, universities etc, which you can upgrade to improve your troops, mages, and even the AI of your mages.
Sadly these Indy's are very rare and hard to trigger, so the chances of you seeing them in a normal game are zero
Although this is intended, as they are in fact one of the many hidden Easter Eggs Illwinter has put in the game. But the good news is you can trigger them if you know how
and luckily for you all, I do know how
As what you need to do is....
Start by empowering a Pan to N10B10, make him an assassin with a Black Heart, and then (from here on things get tricky) every turn for 10 consecutive turns you need to Hellbind Heart an enemy mage 1st round during an assassination attempt (Note, it has to be the first round, or the counter resets). But it is also essential that the Charmed mage (Note, it has to be mage of some sort) must survive the battle against the enemy forces in the province every time. If the mage it killed, the counter is reset. (*Hint. This is why retreat orders are followed in some assassination attempts, but ignored in others)
Then after you've done this, you must send the same Pan to the Arena for another 10 consecutive turns, where he has to Charm (Note, you must not use Hellbind Heart) 3 commanders each Arena contest, all in the first round, and all 3 commanders must make it back to your capital the next turn from wherever they end up (*Hint. Use Vortex of Returning). Like the first part, this process also must be repeated for 10 consecutive turns (*Hint. You will need at least one Wish caster to wish for death matches. I personally prefer empowering WraithLord's to S9 when I need a Wish caster)
And during the 20 turns that all this is happening, you must have placed a stealth army in every province on the map, and the total stealth value of each army must be a minimum of 1,000. None of these armies must ever be discovered either, as if they are, the counter immediately resets.
As Easter Eggs go it's a tough one to get. But well worth checking out IMO if you have the spare time.
Oh, and I also have to mention that to trigger this Easter Egg you must be either EA or MA Pangaea (obviously), playing on at least a 5,000 province map, be the only human player, and up against a minimum of 15 AI nations. With all the nations in the game having to survive from the start to the moment you trigger the "I live in a fantasy world with no connection to reality" Special Indy Attack (to give it it's full name). Plus all of this has to be done within 100 turns.
Good Luck
(ps. I would attach a save file showing this Easter Egg, but I won't, as where's the fun in that for you guys)