The basics are pretty good, but the balancing really needs work.
The yellow faction is absolutely, ridiculously, overpowered. they have the best basic units, the best unique unit and on top of that they get a free bombardment. I can win with all the factions because the AI isn't the best, but with yellow I can do it in less than 10 turns. Oh yeah there's also something about extra prestige for some reason. Especially orange, you can make them fall completely flat on their faces by bombarding their unique unit turn 1 if you're willing to deal with the blue army on your doorstep the hard way.
Blue really has nothing going for it, their units are weak, their ability is more a downside than an upside and they are obviously badly positioned between the other factions. I'm honestly incapable of breaking yellow or green with them (okay I could probably beat green if I didn't have to constantly send troops to keep yellow back), all I can do is take out orange and then hold on to the borders for a prestige victory and that only because the AI keeps thousands of units sitting around.
Orange could have been good with the way their unique unit snowballs into creating more of itself, but the AI being very passive hampers it because everything quickly becomes defended by units sitting around. Maybe if it started with a few more of them?
Green feels like the most balanced faction, though any competence from yellow obviously destroys them, being the one faction that doesn't suffer from attrition. Whether to move or to seed is an interesting strategic decision.