So I'm planning on walling off hell in Bastiongate, Wirejade style. How did you do it? Crossbows everywhere? Live demon trap so no more wander in? Sheer dwarfpower?
Mostly by
exploiting utilising peculiarities of demon pathfinding. While the big wave which spawns when you breach hell does actively assault the fortress, the ones which occasionally wander in from the edge of the map usually don't. Sometimes a group of demons appears on the other end of hell and stays there for a few months doing nothing. Or one leaves the group and goes straight for your masons. Or they spawn right next to whatever part of the wall is being worked on at the moment and force you to evacuate everyone immediately. Or they seem to be doing that, but actually just reach a different corner of hell and leave.
As a general rule, most of the time the demons will chill where they spawned, until something wanders into their line of sight or, worse enough, attack them. Like a dwarf deciding taking a break next to the demons is a fun idea. Or a hapless hauler trying to retrieve something dropped during the last evacuation. Not that they can't get up and attack for no discernible reason at any moment, so their position must be constantly monitored. Leaving the game to run in the background is likely to result in ragequitting. Keeping the military with heavy ranged support is a good idea. Sticking artifact furniture near the spawn points is a good, if a bit exploity idea, but the demons can still miss it and go for something way more squishy.
By the way, you inspired me to cage at least the baby animals (the last time I tried caging adult ones they stopped procreating). The framerate boost is very definitely noticeable - I got up to 15-16 FPS at some occasions, compared to the usual 12, maybe 13.
Now what I'm wondering is how on earth you keep your dwarf pop relatively stable. If child births are a once a decade occurrence, do migrants make the dangerous journey to wirejade still?
It's only stable if "slowly declining" counts as stable. There have been no migrants or dwarven caravans for over 50 years.
Here it began. Putting it shortly, an incredibly obscure bug which occurred when the last caravan was leaving is the root cause of all this. That's the entire reason natural population growth is absolutely vital.
Live demon traps are as far as I know the most common way to do it, but setting things up so that you can ballista-spam certain areas if necessary also works.
I never had much luck using ballistas (ballistae?). Too much effort for too little gain if you ask me. Crossbows and fortifications are more reliable.
One interesting solution is massing war animals. Demons are mighty, but their numbers are low. They will only target one creature at a time and usually pummel it into a bloody pulp, and only then move on to the next one. Meanwhile, the rest of the animals can attack with impunity. Many of their attacks will miss, but eventually they should bring down anything not made of metal. With massive casualties, but hey, they're expendable. Unless the demon is a fire-breather or has deadly dust, those may be difficult to defeat this way.
A group of Slush Banshees is trying to destroy a floor hatch they can't damage, and as long as they stay unprovoked the dwarves can keep working in peace. Thanks to that the northern wall is almost finished, and that means I'm about half done with walling off hell. I will have to kill them off to clear the area for the southern part of the fortifications, but that's a while from now. The constructions in the east should be safe for the most part. Thanks to a ridge there I don't have to build such high walls, so it's hopefully going to take far less time. The wall in the north is 5 z-levels high in some places.
Three dwarves died of old age. All legendaries, which kind of comes with age - one a metalworker. I have more, but it's still a loss. Two miners, one legendary, died in cave-ins while working on what is now the obsidian farm in the magma pool. It's not easy to spot one-tile holes in muddy obsidian floor, and any collapsing tile opens a hole straight to the magma sea. At least it's working now, I guess.
The casualties were partially offset by two new kids. Same couple as before. Also one dwarf broke quite a few bones when someone deconstructed a bridge beneath him, but he got better.
The military saw some action thanks to a minotaur. It didn't take long - a spear to the left leg, a hammerstrike to the right, and another one to the head. Plus a few more in between, not that they matter.
Three adamantine artifacts! An animal trap, a crutch, and a bucket. Oh well, at least I have a nicer well now. And now that my GCS is dead I no longer have a silk farm. Time to get used to cloth again.