On catapults losing to everything in the open field, does that really make sense historically? I thought that there were various anti-troop siege weapons? Onagers, ballista, that sort of thing. Then again, maybe it's a matter of scale. The animation for CR promoted units shows siege towers.
On bronze cannon, yeah, bronze is far better for cannon than iron, iIrc. But didn't steel step it up a notch? Speaking of bronze, it would make sense for it to require both copper and tin. From what I remember, bronze was actually stronger than iron at the time (depending on what alloys were in the iron), but was much less used due to the sheer inavailability of copper and tin.
There are a lot of other historical inaccuracies unrelated to warfare, too. At the end of the day, you just have to remember that it's just a game.