When assulting a top of the line castle with arty towers etc why is it that i cant do the following:

first use all siege weapons soly on attacking arty and arrow towers and then stop the battle and have another go at it the next turn. The towers should be gone then wouldnt you say.

I tried that as a almohead against Bysantium. One army consisting almost entirely of siegeweapons attacked, but the next turn they were rebuilt