The enemy advances on your castle. Those damn ladder-carrying units in the front. Even though your archers are firing like mad, there is a big lump of those guys moving towards your walls. You retreat your vanguard within the castle and let go one of those large logs. It rolls downhill and squashes the whole ladder-carrying lot of them as well as a group of enemy archers.
VERY satisfying :)
Another very satisfying thing is to be able to throw over enemy ladders just in time. Seeing those slow, yet horribly tough armored enemy knights run from ladder to ladder and kicking those over just when they reach it. Take that, little knight! Like shooting (armored) fish in a barrel.
But be aware: It's not all fun and giggles. You will get into incredibly tight spots and the tide of battle can turn very quickly.