Fortresses are new idea by me.
When enabled,they allow the player to see n territories away. n must be bigger than 1. n is at default 3,but can be changed. Also, armies inside fortresses are two times stronger (changeable,from 1 to 3 times).
Payment
They must be paid. However,they can exist both in Commerce and non - Commerce games.
Default price is 10 armies/gold per turn.
Visible and invisible fortresses
The game creator can make fortresses visible or invisible. They are at default visible,but according to the game settings. They are invisible in the far territories in the light fog, expect if detected by the player's fortress.
On the other hand, invisible fortresses are completely invisible, expect for the player that owns them and his teammates. They can be dangerous since enemies cannot know are player's armies in fortresses or not,since fortresses increase the strength of the player's armies.
What do you think?
Have a good day!

Boka B

sounds like you need a mod for that, the commerce mod from TBest already has cities that give special perks.

What are that cities doing?
Have a good day!

Boka B

Building more cities and giving extra armies.

That sounds like regular non-modded commerce

I know what are cities doing,but JK_3 said that they also do something other in one mode. Fortresses are absolutely and completely unrelated idea.
Have a good day!

Boka B