How about even bigger fortifications? Think several lines of defense, all made of interconnected bunkers with a completely enclosed and significantly large above-ground portion.
First of all, extremely thick walls, thick enough to house a row of siege engines, several floors of ammo for the engines and crossbowdwarves to fill the blind spots. Equally spaced towers along the walls contain supplies and several rooms for the dwarves, to make sure that they never have to venture too far away from their stations. Also, they would be the perfect place for control rooms full of levers connected to traps and bridges.
Smaller, elongated bunkers built in front of the main walls and placed in the the siege engines' blind spots, serve as exit points for the defenders, and make sure the invaders "flow" around the blind spots, and not into them.
Flipper arrays located between these bunkers could be triggered by incoming enemies. A large number of interconnected bridges simultaneously raise, sending a bunch of invaders flying back, hopefully to their deaths or at least to a second round of catapult fire.
A great way to waste lots of blocks would be collapsing traps, buildings with a structurally unsound floor that sends enemies flying into a death pit as soon as an enemy pushes a pressure plate connected to the sole support. The enemies fall into a pit full of deadly traps, or into a killing hall staffed by a captured megabeast (complete with a baiting mechanism to stuff the beast back into the cage when required). Another option would be to drop them into a flooded area fed by the aquifier, and trigger a secondary collapse that removes the ceiling and causes the goblin-filled water to freeze.
[ January 04, 2008: Message edited by: AlienChickenPie ]