Draining all the magma into the caverns was a quick way to empty the pit, but not a good idea with regard to FPS. The 9-Z tall windmill farm was demolished, and the micro-water reactors disabled, but FPS has not improved. Sitting at 12 now.
When the wind farm came down, it punctured several Z levels into the fort, destroying the gears and axles for both the water and magma pump stacks, and leaving a lot of openings to the surface. Team Alpha is currently stationed by the breach while the remaining dwarves build floors and walls to seal it off amidst giant undead monarch butterfly, and undead armadillo boar attacks. The farms had to be fallowed, and stockpiles disabled to free up some of the labor force.
Also probably not very good for the FPS was a previous incident where a planned cave-in unexpectedly punctured the floors below, dumping magma onto the memorial slab garden, lighting many of the slabs on fire (lignite was a bad choice of slab material), and releasing many ghosts in the process. Several of the slabs are still burning, but much of the magma has evaporated.
The primary task now is increasing FPS. I have never had a fort get this low on FPS before, so this is new territory for me. First step, trade or atom smash every non-essential item (though I am tempted to preserve an iron shield with 2 kills associated with it). Second step, reduce path finding options. Third step, reduce the cavern layers of undead and flowing liquids. Fourth step, research. A human caravan just arrived, so I'm currently gambling on deactivating the surface breach guards and suspending sealing the breach in order to trade as much crap as possible (mostly old clothing it seems), as 12 FPS is brutal when you're accustomed to 70. I can say for certain that the drop was due to filling a 23 x 25 x 7 pit 100% with magma, turning the top into a variable 3-Z topogrophy of obsidian with chaotic mining / cave-ins, then draining 23 x 25 x 4 magma into the caverns.
On the plus side, I now know how to make a ghost bomb.
Update: After trading away just about everything, atom smashing everything else, limiting path-finding options, and only gaining a handful of FPS, I resorted to DFHack to see if this fort could be saved from FPS death. 10 FPS is not really a workable speed for me to clear the caverns with dwarf might. Especially when every season, "The Fortress Attracted No Migrants"
Reveal showed me the true cause of the FPS drop. I checked the units screen and found over 1300 undead on the map. And in the most inaccessible places too: Very narrow and maze like caverns. There were isolated cavern pockets on the edge of the map, just filled with undead crundles that would be like finding a needle in a hay stack. There appears to be no population limit to the crundle. Surely the total unit count above 1300 was the cause. But I had to be sure. I was about to surrender to FPS death at this point so I figured, what the hell, I'll cheat a bit... I found the biggest clusters of undead and with the liquids command in DFHack, gave them nice refreshing magma showers - dropping the 'Others' count in the unit list down to 470, with the 'Dead/Missing' count up to 1235, and FPS up to 24 rather than 10, with spikes up to the low 30s.
Some further magma cleansing got the 'Others' count down to 163 and FPS up to 88 calculation and 50 graphical (max setting for graphical). This is now a speedy fort again like a fresh embark
Saved by DFHack!!! Now I can continue with the original zombie trap plan unhindered.
The one unfortunate consequence of my unskillful use of DFHack is that the map can no longer be un-revealed. Don't save and exit before using unreveal. You can always re-reveal if you need to. I'm trying not to take advantage of it. Already with hundreds of bars of iron, and having spotted the adamantine when the magma sea was breached, I don't really need to know what's veins are where, etc. Either by design or by luck, large rectangles of normal hidden squares remain over the adamantine areas.
Update: The first Dwarven caravan in years arrived, likely due to rumors from trading so much value to the humans last season, and were also generously paid for their goods. Following that was 26 migrants, a much needed boost to the work force now that FPS has returned to normal. But following that was the arrival of a WereElephant who made quick work of the unfortunate leatherworker by the entrance. To my amazement however, the first macedwarf to arrive on the scene killed the beast with just 4 blows from her *iron mace*. Luck was on her side as the first strike seemed to be very crippling:
The Mace Lord bashes the wereelephant in the neck with her *iron mace*, bruising the muscle and tearing the upper spine's nervous tissue!Update: A dragon arrived right in the middle of the entryway re-construction project. Everything was forbidden / suspended to encourage everyone to get back inside. The dragon scared 1 dwarf who went running away, and then waited around the corner by the entryway, melting him when he came back around.
The cook is apparently unconcerned.
Only 1 of the marksdwarves made it to the halfway constructed tower before the dragon did, but he quickly climbed down to fire 2 shots at point blank range before being melted. Both head shots, leaving the bolts embedded.
Everyone being cave adapted, the mace-lords were stationed just inside the burned down entryway, waiting for the dragon to come in. Taking the gate entrance instead of the wagon ramp access left the dragon in a 1x1 stairway where dragon fire cannot spread as far. Seizing the opportunity, the mace-lords charged in to the stairway, and surprisingly made quick work of the beast. Only bruises were sustained underneath their iron armor as they pulped it.
Update 6/25/19: A Roc showed up shortly after the dragon, but didn't severely injure any of the 30 mace lords waiting for it by the caravan entrance. It did survive a lot longer than the dragon, but also it was a lot bigger than the dragon. Lacking fire breath, I'll take a Roc any day over a dragon. The 30 mace lords seem capable of anything right now, but with 105 dwarves total in the fort, I don't think it would be enough to put down another zombie apocalypse. Lacking flux, I only get about 25 steel bars on a good year from trading for flux / steel items to be melted. Most of them have mostly iron armor. Some patchwork steel and adamantine is being crafted into the mix as it becomes available.
I'm doing my best not to let loose the Fire Devils and whatever else might be in that eerie pit that just got aggro'd. Walling off the gap before grabbing all the adamantine proved to be a good idea.