I want to point a few things:
1. "let's imagine a siege assault where 500 men are trying to take a city defended by 100 men. As the assault continues, men die from both sides, but due to the valiant defense by the city's garrison, the attackers now have 250 soldiers against 80 of the defenders without setting foot inside the castle"
Well, in the related video, we see this Castle Siege where the Vlandians didnt have a chance against this Imperial Invaders, so as far as i know, the Vlandians should (?) be "efficent" as defenders, and their army was not only composed by untrained militia... so this kind of example could happen in the game with the current ia? Or was it an extreme example?
Anyways, it really surprises me how the invaders made such a succesful siege, with almost no loses at all; The invaders really outnumber the defenders, but those should have the advantage of being in the walls so i think it has to be noticeable. Even the defenders are easily taken out being harassed by the Invader Ranged Units. (Looks like they're in Open Field, not defending a Keep)
2. We saw in other DevBlogs that if the defenders really start to loose as the siege goes, they would retreat to the "inner parts" of the Keep
so they can put on some resistance against the invaders, and it would come a "Second Phase Keep Fight" (Something like in Warband) but this "Second Keep Fight" doesnt show up, at least on the Gamescom videos ( Maybe it was deactivated)
We can see in this video how lots of Vlandian Crossbowmen (it's the same siege like the one from the DevBlog) flee to the Main Tower of the Keep;But there's no second fight at all, so how is it works? if they're really few troops, do they surrender to the invaders?
https://youtu.be/GgAMugbWuNE?t=1156
Cheers!