The Art of Pillage
Pillage was, alas, a common form of warfare in this period. An army would march into an area and, while the inhabitants fled for the forests or the local castle, the marauding troops would steal what they could and burn the rest. It doesn't take many troops to pillage, it depends on how large the fiefs population (and thus the number of defending warriors on hand) is. If your force isn't large enough to scare the locals, the garrison will assemble and toss you out. Remember to examine the fief before pillaging. You never know when it might have changed hands. No sense pillaging a fief now owned by your side (your king will not be amused...)
The keep level of the fief is not always the best factor to consider. Check to see if there is a high negative expense value. If there is, it means that there is a good manager there. A typical pillage reduces the fief loyalty by about 20%. The amount of loot depends on your stature (which will decline a bit each time you pillage, the church, and society in general, frowned on pillaging.) You will get more money if the garrison comes out to fight. One long term benefit of pillaging is that the Loyalty of the fief will decline. Thus one player can move ahead of the main army, softening up enemy fiefs with pillages and raids (a quick form of pillage that takes less time, and garners less loot) so that the pillaged fiefs are easier to take.
If you lose a fight with the local garrison, you are retreated up to six hexes away, generally in a straight line. Pillaging, successful or not, takes at least several days.
You can also pillage (after a fashion) a fief without any troops! You can even pillage an inactive fief! You won't get any money, but you can hurt the future owner. Visit the court (if you can, the owner may have locked his courts) and hire away the good NPC's (managers etc). Another trick is to look to see if an enemy player has just entered the game (via the Herald ). If you are close, go pillage their fiefs, hire NPC's before they manage the fief properly. It doesn't take a lot of troops to pillage. There has been success with as few as 600 troops.