Hmm, I've almost finished my first weapon.
It's basically a very tall tower ringed by a number of 10x10 retractable bridges that act as a reservoir. While I pull the switch, it can dump 600 tiles worth of molten rock on the poor fools standing beneath it.
To make it appropriately dwarvish, I've named it Flamecascades the Epiphany of Burning.
It can actually be fired right now - the only part to be finished is a safety device.
The safety device dumps the same amount of water on top of the lava.
I hope the next siege appears on that edge of the map!
EDIT: I have tested it.
I'm... I'm so happy!