One row of serrated glass disk weapon traps and one row of cage traps.
The disk traps are f'ing brutal. I imagine the sharp-edged glass swiping at the invaders, severing not only arteries, but limbs and heads as well. When they make it through that--by stepping on the bodies of their allies, or their body parts, like gory stepping stones--they get trapped in cages.
The trapped invaders get thrown from ten stories up. Survivors are granted their freedom. Any goblins (or dragons) that make it through the traps are given free reign to slaughter until the soldiers can get on the scene.
Seriously, how could it get more fun than that? I put everything on hold just to watch the arc of a goblin's toe or head as it flies gently through the air. Picture, if you will, dwarfs going about their business, carting booze and stone from one place to another; when suddenly, a torso lands in their midst, lobbed over the fortress wall after a goblin smacked the ground at 100 mph. My current record is an eight story bounce. I'm hoping that when my murder pit is ready and I have 31 z-levels to work with, I can at least double that record.
Sometimes I just send my 6 (now 5) champion swordsdwarfs straight to the oncoming hordes at the edge of the map. They are unstoppable, and even completely shrug off arrows and bolts. The only death happened when one lone swordsdwarf tried to catch an arrow in his hand, broke a few fingers, then paused long enough to marvel at the novelty of "injury" to be pounced on by the goblins.
Challenge is for the weak. My Fun comes from disembowelment, dismemberment, and general disregard for the sanctity of life. What else can you expect when my dwarfs regularly mix in ground zombie meat into their meals?