Useful Tips for Sieges
Best units for siege attacks are archers by far. Try to kill as many defenders as possible with arrows before you send in your soldiers.
You should also kill their archers, you can also use cheap units like Yari Ashigaru in loose formation as meatshield if you dont have superiority in ranged units.
AI is not smart at targeting and will always shoot your first unit to get in range. If you are new to the game you maybe dont know that soldiers can die climbing up walls. The higher the wall the higher the chance for them to fall.
So rather climb 2 small walls rather than 1 big. It can be smart not to push from too many side if you dont have like a super high numerical superiority. Get 1 unit up first and let it protect the following units when they are climbing.
Also morale is a big thing in sieges. Your troops can route really fast, so if possible keep your general as close as possible and use stand and fight if you can for an extra boost.
Just remember that auto resolving a siege destroys the gates which in turn disrupts recruitment on that turn. Very important if you are playing a legendary campaign, and trying to keep revolts under wraps. Also another tip, wait for the enemy to turn there back on your archers while they are garrisoned inside the fort/castle. It does way more damage ratio per shot.
Otherwise, there are Modded Maps: like “Better Maps 0.45” which really favor the strategies of “Gate camping” due to the well-made deep water trenches which highlights the importance of controlling bridges and defensive points, in addition to have real capital cities with immersive content to enjoy during the Battles!