The code-writing developer doesn't like AI programming.
As someone suggested, playing maps with pre-placed castles AI can conquer would help. And while the AI won't be able to do everything, it will still prove quite a challenge. Especially if you play one of the races that depend more on armies and less on magic. In multiplayer most national units become worthless in few turns, but in single-player I find using them more rewarding than bLasting enemies by magic.