I am noticing that the AI is not trying to trade with others to get for example metals.
For example, in the game I play right now, I noticed that Irane was training spiders, but she had much stronger designs already built. I then noticed she didn't have any metals, so I traded some metals with her for money (she has more than 100,000 gold). Then, when she finished training the spiders, she started building a strong unit with war hammers.
So to improve the AI, I suggest that the AI should try to trade metals with others when they run out of metals (same for other materials).