While I agree that the sieges need work, and adding more entry points may help, but one thing I think needs to be changed for sure is some of the terrain placement in the siege layouts.
I notice in some sieges there are spike walls that surround the walls, now while these look cool, half of the attacking force spawns ontop of them, around them, and inside them. Those that spawn in them are eternally stuck, those that spawn behind them can hardly get past them, also becoming stuck.
This is shown below in one of my sieges of Marleons I think-
Not only am
I stuck in the spikes, 80% of my army can't get through the spikes either, thus resulting in a 40 minute battle. And this happens every time I siege/defend settlements like this.
Those spikes are the devil!