[Rathar]

If one is being besieged and the attackers break down all of the walls can you forestall a castle assault with the ritual which adds additional fortification points or will they assault you?

I.E. Do you have to cast that spell while you still have fortification defense or can you add those 200 points to 0?

If you can cast it at 0 what happens to the attackers assault orders? Nullified? Assault anyways and you just built the walls for your enemy? Endlessly stymie your attackers plans?