Firebombing

The mages along the walls used a bit of their mana to block incoming flaming projectiles, while letting ones that weren't obviously attempts to firebomb the city pass.

They were limited in their abilities, and the cannons were their highest priority.

"Shame we didn't have more cannons." Princess Alexis sighed.

Marquis Burton shrugged. "This defence has held for over a century. Granted, we haven't faced an attack on this level, but it has served us well."

Dominic stepped to the window and began erecting small [Area Barrier] spells to block flaming projectiles while they planned. He didn't need to stop them entirely, just slow them enough they fell short of the city, or shatter the vessel, and then the oil would disperse into a fireball in the sky.

The messenger standing by the window with Dominic began to relay messages.