I just want to say, this seems pretty good so far, but the main thing that I really recommend is adding little VN style "updates" depending on your progress along a victory path. Though It'd definitely be interesting if all the victory critera can interact, depending on the faction!
Basically, my first (only, so far) playthrough went like this. I restarted the game 5 times before even ending the first turn, in order to re-read the rules and mess around with making moves a few times. Then I started as the Empire, and ended up deciding to just play minimal defense on the east, while aggressivesy attacking the orange faction to the west right away. With the current AI, that worked out well, since the yellow and green factions attacked each other as often as they attacked me. Anyway, I basically managed to defeat the oranges, only to discover that both my other enemies were fielding armies of 900+, and I ended up just fighting a desperate, never-ending battle to hold the capital. Every time the plant-heads and barbarian maniacs pushed into my territory, I pushed them right back and held firm, trying to hold that purple border. And then, the tide started to turn. Holding Neo-Amsterdam turn after turn, and using the supplies wisely, meant my upgraded cities were turning out units fast enough to really hold the line! Hundreds of peasant militia were slaughtered every round, but we held the line, giving and taking the same villages turn after turn, while oh-so-slowly, the artillery rolled east...
And then I won! Just like that! A prestige victory! It was a complete surprise! I guess 5 restarts wasn't enough to understand the actual rules, because I had no idea I was about to win!
Anyway, it was actually a really unique experience compared to Civilization or whatever! Not actually understanding the victory conditions meant that I was just desperately fighting to do the best I could. Which definitely seems very thematic for the Empire!
I just hope you can come up with some kind of narrative system that can pick up on things like that, and feed the player appropriate story bits. Like, the overall gameplay balance is designed to provoke a certain playstyle with each faction. And I absolutely, 100% got the "desperate ancient empire struggling to hold onto their last vestiges of power" from my Empire playthrough!
Oh, one last thing, the "next turn" prompt kept coming up after the game was over. I never saw any kind of definite game over or anything, because I wanted to see what would happen if I clicked that instead. And all the enemy turns just kept going invisibly. Oops? I have no idea if there are actual endings yet!