Game is great, but I have one big gripe with it. The AI is far too predictable, and the perfectly logical choices it makes aren't always so. For example Amphipolis. Once I have the city the Athenians at Thasos will try to land there twice or thrice each summer, only it does it in the least intelligent spot and every time. Technically it might make sense to land on the Western shore, since it's only close to one city, but when there's a narrow pass 7 grannies with brooms could hold against the might of Macedon itself the nobrainer literally becomes a brainless act.
But, my suggestion here is that rather than improving the AI first you could introduce fuzzy logic to it's decision trees. It needs a slight upgrade anyway, but this could give it great gains for far less cost than trying to teach it more tricks. Simply make it so that it considers a few choices for every possible move, and usually it does the smart thing and goes for your valuable cities. However making it choose a bit more carefully you could make it raid poorly defended cities every now and again, thus forcing a player to abandon the carefully placed reaction forces and rather spread out a bit more. That or risk loosing cities.
I know I frequently bypass the front line cities and send a cavalry unit or three on deep raids, and if I bring my companions I might even snag a city just to be an arse to the AI. Make it do the same to me and the dark side will indeed have cookies :)