10th-11th century would mean even less horse armors. Less archers too so less need for those armors, at least in West.
And of course 10th-11th century they didn't use pikes or glaives etc. Lances wouldn't be couched lances of later time. Instead they would be held with a one-handed over-the-head grip. Longbow as weapon of war was late 12th century thing too.
So if Bannerlord is about 10th-11th century, there is lot of things that go wrong. So based on that I would think that it is about later times. Man from 13th century could use arms and armors from 11th century, but for a man from 11th century it would be lot harder to find equipment from 13th century.
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Well considering that there isn't actually any plate armors in the Native game the setting has to be before plate armor became common. I would say probably somewhere between 1150 and 1250 AD so what is that 12th-13th century. Plate armor wasn't really used until the 14th century.
As far as horse armor though, it was pretty well developed if you ask me. It wasn't gothic plate but it was full chain, lamellar or scale. Also as strong as bows or crossbows were, many studies have shown armor from this period was pretty effective protection. For example a Cataphract from this time period might have worn a full breastplate of Lamellar over a riveted chain hauberk over a gambeson. it would take a hell of a lot of force to get through all that and still penetrate enough to be lethal. You also have to consider a horse is a much bigger animal and something penetrating 2 inch though horse armor probably wouldn't drive deed enough to actually reach anything vital on the horse but would probably kill a man.
I guess my point is that I don't think bow and crossbows were quite as effective at actually killing well armored individuals or horses as many think they were. I think they were excellent at killing the lightly armor levies and regular troops that often made up the bulk of armies and were pretty good at wounding the more heavily armored elites, including horses, especially since even the Elites of this time tended to not wear as much protection on legs and arms as they did their core. For example, an arrow than couldn't penetrate the breast area might penetrate 2-3 inches into a more lightly armored bicep and it would likely render the arm useless which would be effective without being lethal.
In any case, I think heavy cavalry was much more effective than it is in game.
Just had a theory regarding armours and the experiences some players are having with them. Torso armour scores can get pretty high, easily absorbing at least half the damage from the most powerful weapons in the game and absorbing the damage from most of the rest. However, Arm and Leg armour scores are drastically lower, more approximating the mid-game levels of Torso armour, at best.
Are people aware of where they are getting hit? I definitely think especially Leg armour could use some improvement, especially as we can wear lower-grade mail on our legs but nothing to match later, more advanced mail armour ratings. The same applies to Arm armour, but less so since a lot of Shoulder and Torso armour overlaps the Arm slot.
Is it possible this is where the experiences around the armour being too weak is coming from? Certainly, my playthroughs usually involve me having at best a score of 20-25 coming from Leg armour, while my Torso armour can easily go above 50.
This honestly goes along with my point how arms and legs are much more lightly armored and it might be true that a lot of the damage units and the player receives comes from damage to the legs or arms. The problem is that this is a HP based game so if you take enough damage to an arm, you still die while in real life, taking 2, 3 or even 4 arrows or cuts to an arm or leg might just be painful and debilitating but not actually kill you or take quite some time to kill you.