We can only hope that they implement something like "raze city" in the future. It would feel immensly satisfying to leave a trail of ruins in the wake of your army.
This would also serve to make defending sieges more exciting as your kingdom would simply vanish from the face of the earth if you lose. To give the defending player a better chance of staying alive you could create a minimum of men required to raze a city (I bet razing a city/fortress is hard work). This would help a vastly outnumbered defender as he could stay alive by using his soldier efficently, and thus making sure the aggressor has too few soldiers left after the battle to actually raze the fortress.