I remember when I first got it, I found it difficult to beat an easy computer opponent. Now I find playing against expert computers too easy.
After you play it for a few years (like me), you get to know the strengths and weaknesses of each unit, and how to combat any enemy you meet.
E.G-Beating a Tiger with nothing more than an Infantry company commander tree and a few Rifle Squads, Sticky bomb the Tiger to take out it's engine so you can call in the arty.
Simple.
Though this is getting more as an advertisement for the game so I'll get to the answer.
I played AoEII and III, some of the Blitzkrieg series, ad others that I can't put me finger on.

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"This Guy Fawkes bloke, do we let him off or what?"