I had a battle in Jerusalem of all places that's quite memorable.
First the setting: A Crusade for Beirut held by the Shia Caliphate that stretched from Tripoli to Armenia to the Horn of Africa. Because of this formidable foe, for 50 years no Catholic power was really able to make any inroads and it was basically a 'forgotten Crusade'; everyone knew the Pope wanted us to do it but the AI and I kept putting it off until we were stronger
Then after conquering Croatia, I made my very-high-martial heir King of Croatia without realizing this would make him fully independent until he inherited my lands (Forgot I was a 'mere' King for a moment). I decided to roll with it and let him be independent for a few decades. Because he was 'Brave' and had no mortal sins, he was a 'True Christian Knight' in the eyes of the Church, so it's fairly fitting that after just a few months on his throne, he declared war on the Shia Caliphate and called me to war! But my blood irrigated courts over all Europe. Soon the King of Bohemia and the Grand-Duke of Rostov answered my call to arms, along with a powerful Catholic Byzantine Dux. Then because someone was finally doing something about Beirut, it's like all European AIs suddenly realized there was a crusade going on: Leon and France declared war as well and the earth shook across Asia minor as doomstacks of Crusaders descended upon the Holy Land.
But the Infidels werent defenseless. At one point I counted six separate infidel armies in the low 5-digit! The first wave of levies I shipped from Sicily were beaten back with heavy losses. But I raised more, added mercs, and raised the Holy Orders and sailed to Acre again.
Soon after, 15000 troops of mine, including 7000 Knight Templars, led by my King and his Marshals and including a particularly skilled Grandmaster of the Order who totaled 65 Martial between them, were hungry and thirsty in the scorched sun (suffering attrition). I hoped to just assault the forts with this massive stack and move back to the coast. I figured I'd be attacked by one of the enemy doomstacks, but I figured my 5000 edge and my crazy generals would do the trick. Except another enemy doomstack marched in from the edges of the fog of war.
Soon, it was 14K vs 20K, but I knew if I fell back to the coast Id take heavy losses and had no troops to reinforce. Then coming from the North, my independent son and heir, King of Croatia and instigator of this whole mess, force-marched 5500 reinforcements into the battle! Phew! Between our high stats, or perhaps because we rolled lucky, we simply shattered the infidel armies despite the numerical equality. Barely 3000 of them escaped while 15000 of us survive, and they were overran weeks later in Outre-Jourdain. My warscore jumped almost 25% from shattering these huge enemy armies and assaulting Jerusalem, and suddenly the Crusaders were winning all over the place after 7 years of inconclusive mutual devastation. This battle alone made sure that my Kingdom, rather than any other crusader's, would keep the spoils of the Holy Land. (Also because when I got called to war I declared my own crusade instead of just accepting the CtA)
Fittingly, my heir to be was rewarded by the Pope for starting all this as well, earning the Crusader Trait. Dozens of years later, after taking over my lands and winning two other crusades, that kid would be the crazy bastard who would engineer the fall of Byzantium as described in my Invasion of Byzantium thread
