The Catapults are inscribed too well. The attack Sam made took a lot of energy. There are tens of Catapults surrounding the city and only a few of them are activated for now. The flame is the buff provided by the catapult itself.
No matter what was placed in it, it would definitely have the same effect.
If Sam has to destroy each catapult, like this, he would be exhausted and he already drew attention from the enemies and the more he focused on the Catapults, the more he would be targeted. The troops are too many.
Only on this side of the town, there are around dozen Pre-transcendent stage cultivators.
In the whole town, there are no more than five dwarves who hit that level, and Sam and his friends comprised of six more. So, only one side of the troops is stronger than the town in terms of numbers.
Them along with the Catapults, if the dwarves really act like they are doing now, they would be in even deeper trouble.