I don't know, I'm by far not that skilled and Violentlash is in its 63rd year now...now tantrum spiral in sight so far, and even the scariest creatures to be found lurking in the darkest darkness of darknesses have met my ways of defense with little resistance, but lots of confusion and spikey deaths...
Then again, I both play(ed) this fortress quite carefully and pretty much on easy mode - it is not in an evil nor savage biome, there is a lot of metal around, tree en mass, and constant rain (I just recently learned that non-sunny weather doesn't cause dwarves to get annoyed by the sun - it rains 95% of the time in Violentlash...lucky me?! Also seems to be a good way of cleaning dwarves, just make a burrow outside and assign it as a civilian alert). Oh well, probably nothing to brag about overall, I still like the fortress...
I'm about to try out a new way of disposing of invaders though, maybe this will be the final nail for my good old Violentlash.
(Overall the most dangerous/deadly thing for dwarves in Violentlash are minecarts.)
EDIT:
Oh, btw, the human caravan remains still occupy my trade post. Their Yaks had calves. I fear I'll soon be overrun by breeding Yaks...Not really sure what to do about it yet, if I attack them, I'll probably not just get Goblin and Elf raids anymore.
OH! And I got a ROC!
A female one. Now, just to get a male one aaaaaand...
EDIT1:
The weirdest thing just happened...
Haven't used my defense yet, but somehow two of the merchants attacked my marksdwarves I stationed near the trade depot to have them off the towers so they'd not spook the invaders. And then a minecart SOMEHOW smashed into them, even though they were on the highest level of my minecart track...which cause the dwarves to smash into each other ... lots of skidding over the floor, a few hands and legs getting smashed, one liver getting torn apart ... all that fun stuff. I just have absolutly no idea why a minecart would smash into that right there. It makes no sense, especially not when seeing that the wounded marksdwarves were all scattered around on the highest level, I could've understood it if they had been flung down the ramps or something...just...just...what the hell.
Well, really hope none will suffer long term effects or something. First time I really have more than 3 dwarves in my hospital, makes me realize I might want to expand it a bit or something.
Oh, and I just slaughtered all the other merchants. There's a strict law against harming my dwarves in my fortress, it is punishable by full use of military force.
EDIT2:
Defense worked. A snowy scorpion (let's just leave it at that) has just dealt with my goblin invaders and is now heading "home".
I would say this is both the scariest and most dangerous way of defending a fortress. I really don't feel like it is worth it.
So, I have a tunnel going from hell all the way up to the surface - just in case I ever felt like releasing hell onto the overworld, that's also the reason why I paved over every square; trees have the tendency to cause cave ins which might smash into my fortress and all, so I don't want them growing anywhere near where I have my fortress.
I now decided to dig a second tunnel going right into the area where I trap invaders with many cage traps. Goblins have the tendency to stop as soon as the first goblin of their small groups has been trapped/killed, causing them to just sit there. When all or as many as possible have entered the room, I close it up behind them and open the doors to hell. Now the waiting game starts, as the demons are quite lazy ever since I slaughtered all that came from there the first time I opened it. When I see one approaching I seal off the room's access to my fortress (which's way is littered with traps, for the invaders). This way the demon doesn't get to my fortress, but still keeps running into the invaders, quickly disposing of them!
As I said, most dangerous way of defending your fortress ever. Just don't forget to close those bridges...
EDIT3:
Eh...he didn't head home. He's now sitting in the tunnel. Oh well, I can seal him off and just keep him there for when I need another round of invader disposal.