In my current campaign I am playing the Scots in the High Age on Expert difficulty. After some grueling battles and the arrival of William Walys, a natural 5 or 6 Command General I had conquered britain and invaded mainland europe along the french coast to try and take the steam out of a rampaging French empire.

A series of difficult battles followed, as my forces battled with the French garrisions and relief troops. Years passed and slowly French territories sucumbed to sieges and fell. I quickly dsicovered the same thing many others will have. A huge stack of troops is great to take a teritory, but rapidly gets dilluted while trying to defend your recent acquisitions.

this leaves the invader with two choices:

1. Leaving a skeleton crew to keep the populace under control and taking the bulk of your army off to continue the invasion.

2. Stopping and waiting to build up a few more troops from the conquered province to act as it's garrison.

If you take path 1: You risk losing the teritory you just gained, as your main army is tied up with a siege elsewhere and the enemy can focus it's forces where you are weakest while you siege their castle. Unless you split your forces up, making both groups weaker, and one bereft of good leadership you are vulnerable.

If you take path 2: Your invasion moves at snails pace and the defender has plenty of time to gather his forces to repel you. You lose the momentum of your surprise invasion.

Now, tot the point of the thread:

While conquering France, I noticed their King was in a bordering province, with only moderate defenses. I invaded the area with a large portion of my army, and viola! They abandoned the province without a fight! I took their teritory just like that, not a drop of blood spilled! Normally they'd fight, or retreat to the stronghold to force a siege and keep me tied up.

That was when I realised why they didn't do that, their King would be trapped in the castle under siege! Knowing they'd lose a battle, and unable to retreat to the castle for fear of their king being trapped, they simply gave me the province!

Shortly thereafter, my treacherous allies, the Castile Leonese turned on me and invaded some of my lands. After repelling them, but losing one teritory I noticed their king was also nearby. I repaeated the same tactic, droping a large force into their Kings territory. They also abandoned the province without a fight. I'm thinking of playing a game of Catch the King, having my main army follow him from territory to territory until his forces gather sufficiently to make a stand, then call off the attack and consolidate my gains.

So there you have it, the best way to conquer groud quickly, without sieges and without having to storn the castle. Hunt the King, it effectively removes the "Retreat to your stronghold option".