Let's go over it.
You start off with smaller funds, which forces you to take garbage units, and a much smaller deployment zone, meaning any effective use of cavalry in defending a siege is out the window. This alone means that you'll be fighting at a strategical disadvantage, unless playing against a total noob. Now you may think that the fortifications and towers justify the smaller funds and deployment zone. So to my next point.
Walls and towers are total crap. The only useful piece of fortification is the Tenshu and the gates. The Tenshu for hiding your units from enemy fire and the gates for being funnels, which can be exploited by yari ashi. Even then, you can just climb a short wall and outflank them. So why do towers and and walls suck?
Let's start with the towers. Towers do very little damage, have very low health and their placement is just bad. More often than not they get deleted by two volleys of any artillery fire. What's worse is that as a defender, you have to play for time, but that's not easy to do when all your outer towers have been captured by the much larger enemy force. "But you said that towers do very little damage" I know I did. But because my units are inferior in quality and numbers, the effects of prolonged tower fire is much more pronounced. Not to mention that the towers actually have a better angle WHEN SHOOTING AT THE DEFENDERS!?!? I'd rather just not have any towers, as any competent enemy will capture them right away, which basicly gives them an elevated ranged unit with unlimited ammo.
Next, the walls. The walls offer no protection. Instead they act as your cage, in which your enemy will be constantly poking and annoying you. You might think that mounting ranged units on them is a good idea. It's a great idea! That is if you want to slowly watch your units being whittled down by enemy fire, as your wall mounted units cannot kneel when reloading (they'd much rather stand and take 5 arrows to the face) and they can't be spread out similarly to a loose formation, which means they receive more damage. Furthermore, dismounting and mounting walls is slower than moving back and forth to avoid fire. So the enemy can disengage and reposition much easily, giving them the ability to isolate ranged units and pick them off 1 by 1, which means that later in the siege your units will be under constant pressure from enemy bows. Remember, the enemy has better troops and more of them, so most of the time your ranged troops will be weaker and outnumbered 2 to 1.
The last thing I'd like to mention about walls, and I'm not a 100% on this, is that every missle unit's accuracy drastically increases when firing upon wall mounted units. The stat doesn't change if you mouse over it. What I mean is that on a lot of walls you'll see your units being protected from the torso down. So you'd think that because 1/2 your unit's body is protected by walls, they should be harder to hit. But no. The second your unit sets foot on those walls, the enemy bows become better headshotters than a CoD quickscoper.
So just put everyone in loose formation and pray for the best.
I'd just like to mention a few random things that don't make sense to me about sieges. I'll try to keep it short, as this is already way longer than I wanted it be.
Any unit can climb a wall of any size. This itself isn't such a huge deal. But when you factor in that your units cannot throw anything down on the climbers, or even shoot at them for that matter, then it becomes a problem. There's morale loss and exhaustion that come with climbing walls, but most of the time the enemy general will be sitting next to a wall using Stand and Fight, so the wall climbing attrition really isn't that bad, especially when compared to the benefit of outflanking your enemy.
Can't place molten lead (or something to that effect) on walls. Can't place artillery on walls. Can't place any deployable defenses that could turn the tide of battle (spikes, wooden barriers, traps, fire pits etc.)
Defending siege battles in a campaign can often times mean defending the fort with a small force, while your reinforcments join the fight. Why is this not possible in multiplayer sieges? Not only would it give the defenders a chance at flanking the enemy (which is impossible in it's current state), it would make for much more interesting 2v2-4v4 battles. Having 2-4 players clumped up in a tiny space is not fun.
FoTS armies as attackers are so OP. Armstrong guns are probably one of, if not the best artillery in the game. Another thing is if the FoTS army can find a decently elevated spot, they can just put their Tosa there and shred anything that dares to step into an outer layer. As a vanilla player I find this very unfair. I don't mind facing FoTS armies on land. You can atleast have a chance of rushing them down. But in the prison that is the fort, it's impossible.
Any feedback is welcome. Right know I have around a 100 ranked games with a 60% winrate and my placement is at around 500 in the leaderboard, so I wouldn't call myself a complete noob, but I may be completely wrong in this so feel free to correct me