The Vampire one had me wondering, if you do a co-op campaign with someone playing as Von Carstein... how are you supposed to deal with his gigantic monstrosity early on? I'm gonna assume no one starts next to each other or something.
Dwarfs and orcs start pretty close to each other, but I think vampires, empire and chaos are somewhat distant. I'd say that the method to dealing with monsters is to either use an anti-monster unit like slayers, other monsters, or siege engines.
Now that you mention it, I think the other guys start with OP units too, not just Von Carstein. I believe the Slayer King starts with Slayers and Kholek Suneater starts with Dragonogres.
BTW, here are more misc info that I picked up here and there:
1. There's no army limits (like the old days). Instead, each lord increases the upkeep costs for all your armies by 5%.
2. All settlements can be upgraded with walls again, like in the old days. However, there are no battles in un-walled settlements and it will instead revert to a field battle if you do fight in one.
3. You can block artillery shots with flying units.
4. You can significantly upgrade the settlement garrisons now, but it costs a building slot to do it. We're talking big settlement defenses though with really strong, high tier units. It's probably because of this that they removed the army cap again as capital cities basically always have a full stack army on them and you can do the same with any other settlement if you want.
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Edit:
I just realized something very tactically interesting about the new sieges.
In the past, there was very little reason for you to leave the gates yourselves when defending against an army. You just waited for the enemy to attack your walls. Here however...
Towers do massively way more DPS than ever before and have infinite range. Even the level 1 base towers easily out DPS anything any other siege defense has ever been able to do in any Total War game. Also, the sieges are all limited to one or two sides now so there's really no escape from them. Lastly, their biggest weakness is that they cannot shoot anywhere except in front of them. So if you get on the walls or behind them, you're safe.
What this means is that you actually don't want them to reach your walls like before. You want them to stay out in the field getting absolutely shredded by the towers. You see what I'm getting at here?
Basically if you have loads of zombies, shitty cavalry, or shitty humans spears bogging down the enemy just outside your walls... the enemy is going to get obliterated. It's not like they can choose not to fight you. The towers have infinite range and you can't exactly flank the crap units as you're probably stuck with big siege towers and if you go around them they'll just attack you in the rear.