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Original AOE2 and AOE2HD both started with the same AI.

AOE2 received a 'userpatch' that updated the AI allowing it to do more complicated things

The userpatch wasn't made my Microsoft so AOE2HD was unable too take advantage of it but 2 months ago there was a patch that made the AI very difficult and aggressive.

Still, the AI is unable to do certain things that the userpatch AI could do (using walls, moving deer, playing on forest maps etc).

The Forgotten expansion ships with a much improved AI. Expansion-less uses the old Petersen AI from the original game, and that isn't particularly intimidating anymore!

You can add your own AI scripts. There is an entire community around AI scripting, if you want good AIs to play against. I believe there are also a few in the steam workshop.