The warehouse that housed the war fires and the explosive ammo was located at the ground level of the fortress and its back wall was the outer wall facing the Verremor army. The number of explosive ammo stored inside was substantial. If all these ammo exploded at the same time, the resulting explosion would destroy the wall.
Since the explosion happened from the inside, the defensive diagram won't protect the wall. Once that happened, there would be a huge hole in the wall. One that the Verremor army could enter the fort from.
The number of defending soldiers inside the fort was not high. If the Verremor army could enter the fortress, then it would be easy for them to take down this fort.
And this was what White Death had planned for. Yellow Death and his two followers were currently standing before the war fires.