I'd actually warn you off Empire. The Battle AI got worse, if that's possible to believe.
As an example, the AI only knows a total of three tactics, all useless. First, it advances on you if you have cannons firing round shot. In the even they get close and you switch to canister shot, they then retreat. This allows you to control their movements by a single button press.
Secondly, they garrison buildings if there are any on them map. Which is sort of okay, except only one man can fire out a window and the men don't rotate, so in a group of about 80 men only 10 or so will ever be firing, meaning that, hilariously, its better to stand outside and just fire in. Better yet, the enemy doesn't react to you pounding the building with cannon, howitzer and mortar until its just about to collapse.
Finally, and this is the AI's "Ultimate" tactic that it will do no matter what, is attempt to flank you with a single group of cavalry. And they charge the entire way, tiring them so much that even a group of town militia can rout them with acceptable losses.
Siege battles are, believe it or not, worse. As the defender there is absolutely no point in manning the walls, as all units have grappling hooks and they will just get scaled and your men slaughtered.
No, its best to put your men in the middle square lined up to face the gates, as as soon the enemy has a few men on the walls they control them, jump back down and try to march in through the gate, leading to a massacre as they march in, get shot to hell, break, regroup, march in, break ad nauseam.
I'd rather advise you buy Napoleon, it seems to have solved most of the issues, of which the above are but a few.