This is the value of prudent scouting. I saw what your pretender did in Arco so I built my armies to resist it. I probed the forts before I attacked them so I new what kind of opposition I would face. Rock-bag-scissors etc.
Hahahaha! Thats great! But there is one little problem with that plan. Your pestilent religion is not all that popular with people outside of your capitol and without faith there can be no fire! I am happy holding of my attack on your capitol for some time. I can always entertain myself by taking the lands your turtling has left unprotected.
As for why the oak? Well fires of such intensity needs high quality wood to burn well y'know.