I loved playing this game back when it came out except for one thing; I hated how spearmen and other ground units could tear down stone walls. Is there a mod or patch to fix this?

Just put archers ontop of the walls, with a few extra spearmen to push the ladders down if the ladder guys try to come over the top. :)

In multiplayer there is an option called Strong Walls, which makes infantry useless against walls (only allowing siege equipment to tear them down). In single player- you'll just have to protect your walls :p