Thanks to all for your help and your comments.
To answer some of the questions, yes I have Kingdoms, but haven't seriously played the campaigns yet. Just dabbled with the Americas.
And by “just standing there” I meant just that. The enemy would literally drop the siege equipment (they just had a ram and ladder), walk a few paces in front of it, and just stare at my wall. Even when I moved a couple cavalry units behind them, they still just stood there facing the wall (but out of range of tower defenses). Maybe it was a bug, because I haven’t seen them become statues as badly as that in a number of battles.
I did install the Darthmod patch. To be very honest, I didn’t like it. I’ll have to take your word for it that the AI is improved, but just seeing the few apparent changes it made on the campaign map, I have to say I liked the original version better.
The ONE thing I did like about it was that you could actually see the movement of your units on the world map before you actually had them move. Meaning after you select a unit you can move your mouse and the path automatically draws before you have them move. That was something I was hoping for on the original. As it stands now, having them move, then pressing backspace to make them stop a bad or unexpected move is a bit annoying.
I currently moved up and am playing on medium/medium, and have to say the battles do seem just slightly more challenging for some reason. Maybe it’s my imagination though

. It does make it a bit more fun however. I will try a hard/hard campaign next, because on easy mode, the ratios of 1 to 25 being killed battle after battle was a bit unrealistic.
Here's to hoping Empire will have some improved AI, because after being introduced to this series, that will be a definite purchase when it's released.