Ok look. I have a defense that so far does indeed remove armies from the face of the earth. It involves traps, fortifications and a death pit.
You have a bigass death pit. You have a series of bridges with 3 (or more. whatever.) + shaped
X
====XXX==== <---like this. X is platforms, = is bridge.
X
platform of land in between. Fill these sections of land with spinning blade traps (my personal favorite). The goblins will either dodge the traps and fall into the pit (which should be 10z levels deep minimum. Spikes at the bottom are a +) or they will die on the blade traps. Their corpses are also usually flung to either side, preventing (but NOt eliminating) clog. In the center of each platform, put a pressure plate to drop the bridges, also attached tp a lever for when they decide to camp the bridge. Now then, the fortifications should be surrounding the pit.
FFFFFFFF
F F
=X==X==X= here is a poor illustration. F is da fortifications. X is platforms.
F F
FFFFFFFF
Fill these with crossbows. They will shoot goblins right off the bridge via goblin dodging. All I have to do is set an alert for a single squad of marksdwarves, and the automated defenses take care of the rest. The marksdwarves also help against trapavoids, since theyll dodge into the pit from bolts even though they dodge traps. Mine has a bait-duck on the other side of the pits, tethered to a chain, as well as a flooding system attached to a river that drowns the pit and drains into the caverns.
I ain't no grand master dwarf, but this works beautifully for me. Shreds funvasions like nobody's business.
PS: put an expendable dog or something on the center platform for kobold duty. Furthermore, a goblin will occasionally koolaid-dodge his way to the other side unharmed. Just put a axedwarf or two, or maybe a swarm of dogs, over there and you'll be fine.
::EDIT::
Forgot something. Having a one-tile wide entrance to your fort is bull. Takes forever to clear the crap away from it due to cank. Here is my solution
FFFFFFFFFFF
F F
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==X===X===X= the - are bridges hooked to their own switch. retract during siege time.
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F F
FFFFFFFFFFF