Yes, the Siege AI is the most glaring failure of this game in my opinion.
Little funny story about it.
Have been playing the last beta version (I mean, beta *beta* version) and I was having a really successful campaign, 30 years old start, vassal of the northern empire. 74 tier 4-5 units after... 35 days, give or take a couple.
I had just recently pledged to the northern empire and joined an ongoing siege, the northern empire was sieging Baltakhand. I joined the fray and the experience I had was ridiculous. They had 210 defenders, mostly militia, while we had 1200+ soldiers with a huge number of menavlion and bucellarii. Our own catapult managed to somehow destroy both of our siege towers... killing at least 40 men in the process. Then the ram was pulverized by a defenders' catapult. Ok, I think, the men will probably use ladders... but wait! They raise the ladders, step on them... then get stuck and not even while taking casualties will they advance further! Thankfully a group of soldiers started bashing the main gate with swords: after a while it went down, our soldiers took the other gate out in short order... and eventually the city was ours. 520 losses.
Ok, now... surprisingly the city is awarded to me of all people! I stock up on recruits, forced villages to give me more men, eventually I could field 300 decent soldiers against an onslaught of 1500 attackers who didn't make me wait longer than three days to come and siege me. With no help in sight I decided to fight. Historically, 300 good men would be too much for 1500 attackers (most of which are either peasants or cavalrymen.)
Anyways, skip to the second siege. Already, the interface won't allow me to separate groups in a decent, intuitive or even working manner so I can tell X infantry to guard the right side while I tell Y infantry to guard the left, or the gate. The archers? I manually place them in two separate groups on the walls. Looks good I thought, and then the siege started. We had 2 catapults and 2 ballistas. I managed to destroy the battering ram moments before it took down the gate by literally throwing rocks at it. Good, I think, now I can tell the other soldiers to guard the f***ing walls instead.
Oh no. The soldiers refuse to move. Most of my infantry decides to run aimlessly (even with the charge orders) from the left to the right stairs without actually engaging the enemy. Those who reach the walls stare at the enemy which they could easily surround and keep from getting on the walls WHILE BACKPEDALLING LIKE MICHAEL JACKSON.
But the archers, the archers must be doing great, right? Wrong. Those on the perfectly placed towers are facing... the wrong side. I teamkill one of them to see if he's used any arrows: he hasn't. Not a single arrow from the right and left tower for the ENTIRETY OF THE SIEGE. The other archers have been standing in full view of the enemy archers (without even trying to use the cover in place for them to take shelter) and are all either dead or staring at the void that is their mind while the enemy scales the walls. All this time, I've had 100 infantrymen told to either STAY ON THE WALL or CHARGE. What did they do instead? They avoided the enemy and ran from a stair to the other. Great.
So at this point the siege is lost - 300 vs 1500, ok, acceptable right?
Only that I killed 35 in that battle. My entire army of 300 something killed 49. All thanks to the absolutely retarded AI.
*Italian chef's kiss*