You can play with the AIs allied. Its pretty easy to do.
Such as my WEvsTHEM scenario where I have the lower end of the map blocked off from being starting locations. And down in the lower left corner is the
Deadly Atlantis Rlyeh Ermor alliance. In the description on my site I recommended starting Ulm, Arcos, and Marignon as human players then playing all 3 as a way to get the hang of multiplayer alliances. If I remember correctly they do start with additional castles in strategic locations. And they will NOT attack each other.
I was going to expand it with a cover story about how each was ruled by a suped-up dragon which were nest siblings but I dont think I ever got it done. If someone wants to take the idea and go with it, that would be great.
And the Random Opponents scenarios at my site also include not only random opponents but also random alliances between them. It could probably be done better but it is there for you.
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www.Dom2Minions.com