The next thirty minutes is spent trying to make the thing work. Pointing it towards the sea, they insert the red cannonball and activate the trigger. An intense sucking is heard, and they immediately take out the wax mold. Looking inside the cannon, they see the cannonball is gone. They do practically everything but holding down the button; they knock the side of it, set up a buttress in from of it, and they even shove some regular cannonballs in it. After a while, my DMPC wizard suggests, "Maybe you should hold down the button, sir?" That they do, until I realize that the buttress is still sitting in front of the cannon. I basically yell out, "You feel the intense urge to move that shit out of the way of the goddamn cannon!" The party takes reflex saves to see if they can do it before the thing goes off. They succeed, just barely, and a red-painted ball bursts out, towards the sea. About 100 feet above the water, the ordinance explodes into a massive 300 ft diameter fireball.
The PC's are a bit surprised, to say the least. They test it two more times, with similar result, and then start heading back to the castle. On they way they're ambushed by a rather large force of elite orkish riders (I think I made them ride huge wargs, just to rip of LotR) and an elephant rider. Without the means to run, they stand and fight, using the cannon to destroy the elephant at a distance. The orks go down relatively easy, and the party continues home.
At the gates, they see the familiar head of the Chamberlain stuck through a pike, decorated with blood and feces. When asked, the gate guards explain that the man was a sympathizer and traitor. In front of the King, they are told the Chamberlain arranged the ambush, but that the testing of such a powerful weapon was much more positive than the treachery was negative. As a reward, the group is given Ring Gates, enchanted Dragonskin Armour (Red), and a heavily enchanted longsword. They thank the King, and are off.