Campaign battle difficulty was on hard.
The fighting might seem at first random but there was a definite plan. The strategy was to use the sally to deter the ram, a practice that works well in Chivalry II, the Sicilian Vespers and in vanilla. Then destroy the ram with long range artillery fire and retreat back inside the fort to defend the walls against the infantry. Most of the Mongol infantry being archers, there was a chance they would be defeated in melee at the walls, and in that way the fort could be saved.
To distract the enemy cavalry, the mounted archers sallied out first, but the Mongol horsemen paid relatively little attention to them. Their ram continued to advance, despite the sally. The Khwarezmian general Nazim Khucedin, despite his lowly one star command level attained in the recent battle against the Mongols, was our best general in the area. Nonetheless, under the circumstances, seeing the Mongols pushing on with the ram, he resolved to charge the ram with his bodyguard. He was countercharged by all the enemy heavy lancers and in the unequal battle that ensued, our heroic leader was slain.
His charge nonetheless gave time to the catapults to deploy outside the gates, fronted by two companies of militia spearmen. As the Mongol siege tower started to burn, the ram became clearly the Mongols’ best chance of making their way in. So the Mongol cavalry charged the spearmen. The lowly militia spearmen held their ground, nonetheless, and after a while the Mongol horsemen pulled back to let their ram move forward. Unfortunately and annoyingly, the ram survived unscathed several hits by the catapults. And so it reached the lines of the spearmen and once the ram crew got entangled in a melee, the Mongol cavalry joined in. So far the attackers were fighting sensibly, hats off to the AI.
Overwhelmed by the masses of the Mongol cavalry, the units outside the gates, already decimated by the torrential hail of Mongol arrows and the previous cavalry charges, broke. Two companies of spearmen kept in reserve inside the gates then sallied out to allow the broken units to rout to safety and to repel the Mongol cavalry and take over the ram.
While this epic struggle was going on, a unit of Mongol skirmishers had managed to place ladders on the northern wall. That was not as bad as it might sound, in fact it turned out to be fatal for the Mongols and even saved the battle. The Mongol infantry consists only of archers. It is their cavalry which poses the greatest threat - and the only way the cavalry could pose a threat would be if they could get through the gates. The Mongol archers, however, had suddenly abandoned any thought of recovering the ram and rushed in one body to the ladders. There were Hashashim on that side that could hold them back for a while on their own, so the spearmen stayed in front of the gate until the Mongol lancers broke off and moved away. That gave the spearmen the perfect opportunity to quickly retreat into the safety of the fort, as all Mongol infantry was now occupied on the northern wall. Not only did they succeed in closing the gates behind them before the Mongol cavalry could get to the gates, but as the Mongol horsemen were retreating, their general, Jebe the Tyrant, was killed by a missile from a trebuchet.
The ram had been abandoned and the only chance the Mongols might recover it would be if some archer unit routed from the wall, rallied and then picked up the ram. With their ten star general dead, self rallies would now be unlikely. Still there was one way to prevent that eventuality – there were yet some Khwarezmian mounted archers surviving outside the walls who, having spent all their arrows, had run out of sight and out of mind. Now they returned and attacked the Mongol archers at the ladders from the rear. Caught between the Hashashim and spearmen on the walls on one side and the cavalry in their rear, all Mongol foot archers broke. They were cut down by the cavalry, as the Mongol cavalry, left mostly with mounted archers (who had used all their arrows), watched mostly from the distance, failing to respond in a determined way. Once the Mongol infantry was completely eliminated, there was no chance the Mongol cavalry (some 2000 left against only about 500 surviving defenders) could use either the ram or the ladders.
Unbelievably, Gul’dursun had been saved. The two key river forts were subsequently reinforced, as the Khwarezmian cavalry was being retrained for the final showdown. Even had the fort fallen, the Mongols would have needed to cross the ford against opposition from Khwarezmian infantry and artillery, then take another fort on the opposite side in a second siege, then finally besiege the nearest city – Urgench - if it was the settlement they were after. By then, almost certainly the Mongols would have lost all their infantry. Yes, there was another stack coming from the North of the Aral Sea and the ransomed units under Khan Subujin, but even they would have to take a fort, before they could reach the walls of Urgench. The Mongols could have not made a worse entry than entering Khwarezm east of the heavily defended Amu Darya. The AI had other options, to invade further south. The Mongols had under the circumstances at least the chance to wear out the Khwarezmian army in open battles, which the AI squandered. At the very least they had the better option to do nothing and stay there being a nuisance, as in Kingdoms Crusades.