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[Bakaridjan]

This is something I've wanted to try for a long time, but never put enough effort into trapping the crabs. Then I accidentally caught a bunch of them in my volcano drain. The initial results are impressive and !!FUN!! So I thought I would share a few thoughts on design, in case others want to try it or have designs of their own that are working well. This was my first go so I'm definitely not any kind of an expert at it.

Spoiler (click to show/hide)

So here's what I've learned so far...

1) If you're going to make a serious dent in a big siege you're going to need more than one or two crabs. You probably want to think in terms of batteries instead of individual turrets. In the pic above, I've got 5 crabs going. Three in the center and two on the right. I'll be putting in another battery of 3 crabs, or possibly more, soon.

2) In light of 1 you're going to need a good supply of crabs. The good news is they multiply only slightly slower than rabbits. So if you can catch a mating pair you probably want to devise some sort of breeding room where they can multiply and then be trapped. They are untameable and unbutcherable, without modding, so plan accordingly. A good supply is also important because you're likely to lose crabs occasionally to enemy archers. I lost one in the siege above. It's probably possible to design more archer-safe turrets than I've got too.

3) Burning enemies make a lot of smoke and smoke can block pathing, so you probably want to have a good open field and approach to your fort. A narrow chute or hallway would quickly become blocked with smoke causing FPS issues and stopping the advance of the siegers.

4) My turrets are similar to what you might create for a GCS silk farm. I pit the crabs in from the top through locked hatches. I build a ramp when I need to pit them and then disassemble it to make it a little harder for building destroyers to climb on top and wreck them. With a good supply of crabs I'd rather repair than build building-destroyer-safe turrets, but that could be done too. I didn't build release bridges/hatches to drop them out of their turrets for repair etc. but that would be a smart idea.

5) Bridges or another way to close off your turrets are important, because they will fire on anyone passing by, be they migrant, mercenary or bard.

6) One +/- about these turrets is that they burn up the enemies and their clothes leaving no corpses. This is nice if you don't like doing big clean-ups but might be a disadvantage if you're depending on invaders to augment your clothes. Also burning enemies bleed like crazy, often leaving long trails of blood until they succumb to their burns. Some players will see as a nice perk, others will pray for rain to wash all the mess away.

7) Lastly, mc turrets aren't going to completely destroy any sieges, some enemies will always get through, so you'll need a backstop of some sort to finish off the survivors.

I'd love to see others' designs.

[Nullsrc]

This was the best possible thing to be reading right after I woke up. It is now my favorite DF design/death machine/killer of things/!!Science!! experiment.

[Magnus]

I made one of these ages ago in Ilrom Ziril: http://www.bay12forums.com/smf/index.php?topic=148021.msg6242752#msg6242752
Had about 15 crabs in it (18 after one of them had triplets), the setup looked like this:
BBBBB
BXXXB
BXCXB
BXXXB
BBBBB

B = drawbridge
X = fortification
C = crabs

The drawbridges were there so I could close the turret when caravans came. Sadly, it was almost useless. Any enemy with shield or a minimum of Dodge skill would easily avoid the magma indefinitely. It DID succeed in burning down half the mountain after it finally set fire to one of my own dwarves who was stunned from sun sickness.

I also built a dragon turret with the same setup. It was moderately more efficient (although the dragon is an abject coward and easily killed by marksgoblins), but it melted the drawbridges so I had to rebuild them after every siege.

The best turret like this I've found to be the Fire Imp turret. Fire Imps are not tameable, but oddly enough they were friendly to civilian dwarves in my game (they did go apeshit on military dwarves though). I put 4 of them in a turret, and while they only managed to set fire to one goblin, the smoke was effective for stunning the rest of them. The drawback is their range, the enemy has to pass right by them in order for them to fire, and like the dragon they are vulnerable to bolts and arrows.

[Bakaridjan]

Interesting, mine seemed much more successful. My crabs do miss a lot, but with a lot of enemies, they also hit a lot. There would often be 5-10 gobbos on fire at a time. Maybe something changed in the code behind it all. The other difference would be that all of my cages only have on crab in them each but they're shooting from multiple angles, and there is something of a cross-fire going on between them. In real life, that would seemingly make a big difference to blocking shots with a shield but I'm not sure DF mechanics are very concerned with how something would work in real life.

Also, you remind me, it's probably not be a good idea to set something like this up in a heavily forested environment. I've got no trees or vegetation in my rocky wasteland so I don't have to worry about lighting the world on fire.

[Innocent Dave]

Also, you remind me, it's probably not be a good idea to set something like this up in a heavily forested environment. I've got no trees or vegetation in my rocky wasteland so I don't have to worry about lighting the world on fire.

On the contrary, nothing wipes out a siege quite like a forest fire.  Flat, grassy areas plus uncontrolled flames tens to equal death for everyone involved.  Just make sure there's no way for the flames to reach your pastures, entrances, etc.

[mirrizin]

I just wanted to state the obvious: That is awesome.

[miauw62]

isn't corpse destruction almost stricly a positive? means your dwarves dont get bad thoughts from the corpses!

[kingu]

I would be interested in your best methods of catching these machineguns?
I have had a couple through the years but I have yet to find a way to catch them because they never seem to want to path to my fortress.
What attracts a a magma crab?

Also, the last time I had one I built a test turret and tried it on a lone goblin prisoner loose in a room. The magma crab was unable to hit it at all. That goblin probably turned legendary dodger bu the time I released it several years later. Do your crabs actually hit or do they just set the ground on fire? My fort is on a glacier...

[Bouchart]

Catching magma crabs is no different than catching any other wildlife: you funnel them into a small area lined with traps.  The only difference is that you need to chancel your magma a good deal from the vent/sea so that dwarves aren't pelted with basalt from magma crabs who just sit in the magma.

[kingu]

Catching magma crabs is no different than catching any other wildlife: you funnel them into a small area lined with traps.  The only difference is that you need to chancel your magma a good deal from the vent/sea so that dwarves aren't pelted with basalt from magma crabs who just sit in the magma.

Yeah but all my magma creatures seem reluctant to path out of the natural lava pools. Maybe I am just unlucky because I have alot less trouble catching regular wildlife.

[Bakaridjan]

Do your crabs actually hit or do they just set the ground on fire? My fort is on a glacier...

I'm not exactly sure why I seem to be seeing better results than others. I am shooting with more than one crab into huge masses of enemies. This siege was over 200 invaders. I'm in a rocky wasteland so there's nothing to set on fire unless dried vomit burns. A few possibilities I can think of again.

1) Something has changed in magma crabs or goblins and dodging.
2) Shooting from multiple directions > goblins ability to dodge.
3) Having a target rich environment dramatically increases the possibility that one gobbo in the magma's trajectory is going to be hit. I'm not totally sure about how that works. Can the flying magma hit anyone beyond the original intended target?
4) Having a target rich environment means that there are always going to be some gobbos who are bad at dodging and therefore they are eventually getting hit (I somewhat doubt this because the first enemy to make it through to my waiting melee dwarfs was a recruit, but who knows).
5) It could be that the crabs are making magma puddles that are inadvertently being stepped in by gobbos, but this would also surprise me.

[Detros]

Do your crabs actually hit or do they just set the ground on fire? My fort is on a glacier...

I'm not exactly sure why I seem to be seeing better results than others. I am shooting with more than one crab into huge masses of enemies. This siege was over 200 invaders. I'm in a rocky wasteland so there's nothing to set on fire unless dried vomit burns. A few possibilities I can think of again.

1) Something has changed in magma crabs or goblins and dodging.
2) Shooting from multiple directions > goblins ability to dodge.
3) Having a target rich environment dramatically increases the possibility that one gobbo in the magma's trajectory is going to be hit. I'm not totally sure about how that works. Can the flying magma hit anyone beyond the original intended target?
4) Having a target rich environment means that there are always going to be some gobbos who are bad at dodging and therefore they are eventually getting hit (I somewhat doubt this because the first enemy to make it through to my waiting melee dwarfs was a recruit, but who knows).
5) It could be that the crabs are making magma puddles that are inadvertently being stepped in by gobbos, but this would also surprise me.

I would go for 3, AFAIK any ammo won't just drop down on the tile the target was it will fly a bit more. Also, if they dodge form one shot, they can easily dodge into the second shot coming from different angle. And also, with smoke preventing pathfiding, once there are few hits there is suddenly much less possible tiles to dodge to.

[Ravendarksky]

Now lets see some HFS deadly melting dust turrets =D

[kingu]

Now lets see some HFS deadly melting dust turrets =D

I have captured those kinds of guns but unfortunately my game always gets unplayable due to FPS after breaching hell.

[Bakaridjan]

Oooohhh! I've got some Wraiths of Flame that make magma crabs look like babies throwing warm mashed-potatoes. All the original ones are killed off, but wanders occasionally come though. Hmmmm, I've got the itch to start a new fort however and trapping them would probably be a real time waster. Maybe another time...