Hmm... That gave me three ideas.
One: Goblin siege is making its way towards the fortress entrance. A lone siege operator rushes to a ballista and loads it before getting into firing position...
He shoots the bolt, which flies out in a loose curve and decimates the goblins (which were attacking from the side of the map, so they're lined up just right). Once it chews up most of the siege, the bolt continues back around, flies through a fortification, and then completes the circle by flying straight back into the ballista, thus splattering the unfortunate operator. The screen goes blank and states a single word: "Legendary".
Two: A dragon is attacking the fortress, and is making its way towards the entrance. A siege operator jumps into a ballista with a lever next to it and fires a ballista bolt at the dragon, skewering it on the tip. While in flight, the bolt trails a string of "§" shapes that connects it to the ballista.
The operator gets out of the ballista and goes over to the lever, which he begins pumping furiously. This winds up the rope, causing the dragon's corpse to be reeled in on the tip of the ballista bolt.
Three: Little picture of a dwarf in a ballista that is surrounded by small chunks. Dwarf fires the ballista, and the bolt flies off the side of the screen. After a short time, the bolt reappears on the other side and flies back into the ballista, spraying operator parts into the predefined "chunky bit" piles. A new dwarf steps up to the ballista.
As you can see, it takes me a very short time to come up with a complicated idea, and a very long time to come up with a simple idea.
EDIT: Four: Stream of siege operators that fire themselves into a wall using a catapult.