Magma-based weapons have declined since the zombies-don't-die-in-fire update. Based on the lets plays that I have read, they were also reduced in usefulness by the 3D version. We really need a way to properly flood a map in lava these days.
Well we *could* channel away everything except the edge of the map so it pools everywhere. But frankly our militia would still be faster....in this battle, even though we lost maybe 4 dwarves (2 civilians, plus a couple of soldiers who were part of the 5-strong group that had to see off about 8 goblin melee troops in close combat after the bridge closed), we wiped out all bar about 3-5 goblins. For future reference, here's how to wipe out a siege with our current defenses ( I may do a diagram later):
1. Make sure we've got livestock pastured in the top part of the outer compound. This is our "killzone", and we need to draw the hapless invading army into position so we can shoot them. We *could* use cheesemakers, but we're short on those.
2. When danger comes, evacuate civilians to the inside-fortress area and the underground area (our alerts are kinda broken for now, I'll see about fixing some of them later), and close both bridges across the moat once people are inside (there's a handy lever room on the surface for doing this)...leave the bridge in the outer compound wall open though.
3. Station marksdwarves in the two towers (I only used one, because DuckThatQuacks never considered such luxuries as 'floors' to be essential to snipers)
4. Move melee squads to near the gate.
5. The invasion will be drawn to the scent of goats and turkeys and stuff, then get shot to bits by the archers. If you're really lucky, some animals will take to the walls and you can wipe out a squad or two as they clamber around up there.
6. When they try to flee, lower the bridge to the outer compound. Once I've got the darn thing rebuilt and linked up, it will also be possible to use the outer-wall bridge to seal the way out.
7. Kill. Everything. This is quite easy, because the enemy is by this stage worn down, injured, vomiting and running for their lives. So it's a bunch of 1v1 and 3v1-in-our-favour battles.
This tactic works wonders for training soldiers in the gritty art of real combat. We gained another elite marksdwarf this time, and a bunch of named dwarves (Urist the Grey II, Sebastian Flyte, Me) got quite a lot of kills...as well as our royal spearmaster, who is honestly slighty scary. When I get the new armor and weapons out of the forges, they'll be even better.
Edit: Oh yes. While it worked OK for me, using the drawbridges to dump entire squads into the moat is not advised.