How effective are fortresses? Or fortified cities?

[Seytan]

Do they force the attacker into protracted sieges? How is supply managed?
Can you sack cities?
Watching the stream he mentions rebellions, what determines this?
I noted in the Stream he captured enemy units, do they employ replacements from the capturing
army to flesh out if damaged? Or is it like ACW where they didn't?
How is winter attrition handled? Are winter quarters a factor in game play? How is it modeled?

Well the Stream answered the siege question, it seems very well modeled. Supply seems like ACW.
I like the option to pick your tactical deployment for grand battles. Well done.

[Leibst]

Supply in TYW is similar to AJE in the basis. Its simple system. You can live from the land but that will pillage the region. So you live of your spply carts. But normally you will find supply lack. And you will suffer more casualties from lacking of supplies than for battle.
There is no sack of cities. It is not easy or possible to simulate this. But if we found how to implement that it will be a nice feature to add.
There is a possible add regarding taking WS from prisoners. But atm is not implemented.
Sorry for the late answer.