If you didn't already know, you can destroy trees with Siege Onagers (available only
to the Teutons, Celts, Mongols, Aztecs, Saracens and Koreans) or Trebuchets. Siege 
Onagers are far more effective than Trebuchets. You can use this to enter enemy towns
and surprise them without breaking down their walls. If you want to be really nasty,
you can destroy all the forests near their towns so they have to go outside to get 
wood, and you can kill the villgers. You need to know this in some campaigns.