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[arka0415]

I'm planning to build a pump that floods my fort's entrance with magma, but I'm not sure how to make it kill goblins versus just scaring them away.

If goblins are pathing into a fort and magma comes flowing towards them, will they just see it and run away? Do they need to be locked in by bars/grates?

[Bumber]

They see it as an obstacle. If it blocks their path, they'll move to do something else.

[arka0415]

Thanks, this is helpful - I assume even 1/7 magma is viewed as an obstacle?

I'm pretty sure magma moves slower than goblins do, even when pumped out of a screw pump, so this means the goblins definitely need to be trapped - either by magma flowing in from behind them too, or by activating an obstacle that shuts them in.

If the magma-pump trap would only work in a room of fixed size, I suppose there's no advantage compared to just replacing the floor with a retractable bridge and dropping everyone in the area into magma?

[Bumber]

As long as the walls are smoothed natural stone, else they can climb.

You could also dig a deep stairwell and pour magma down on top of them once they're close to the bottom.

[arka0415]

As long as the walls are smoothed natural stone, else they can climb.

This is good to know - so if goblins were in an open-topped room with non-smoothed walls that was filling with magma, and no other way out, they'd climb up the walls to escape?

[Saiko Kila]

As long as the walls are smoothed natural stone, else they can climb.

This is good to know - so if goblins were in an open-topped room with non-smoothed walls that was filling with magma, and no other way out, they'd climb up the walls to escape?

That's the question worth experimenting upon. If there was a path accessible by climbing, they would climb the walls trying to escape (some could fail and drop into magma). But if there was no path, then the question is open to me.

I think they would climb, but with all the changes to pathing, it's impossible to know without trying.

[Thisfox]

If the walls have an overhang, they climb up but can't get any further, around the overhang. Same as if they were trying to climb down. They can climb to escape the magma, and then shoot arrows from their vantage point.

To speed up the magma spread, if you have pumping capabilities already, it's best to fill a cistern above the magma room ahead of time, and then gravity-feed the murder holes. This can sometimes cause the magma to go faster than the goblins can run. This also means that the cistern can be carefully sized to prevent overfill killing valuable dorfs.

[Moeteru]

I don't think they'd actually try to escape - at least not immediately.
In fact, from what I remember of magma traps in the last version they don't even stop walking towards the magma until something makes them re-calculate their path. That usually happens when either they get a couple of tiles away from the magma so their next step is blocked by the heat, or enough of their fellow invaders get killed that they decide to abandon the siege.
I would guess that fleeing invaders might decide to climb walls, but if the siege is still active and there's no other path into your fort then I'd expect them to just stand around near the magma, oblivious to the imminent danger.

[Bumber]

I think they would climb, but with all the changes to pathing, it's impossible to know without trying.

They'll automatically try to grab the wall if they start falling, which will prevent them from falling into the magma below. Even if they don't try to escape, they'll still be sticking around until dealt with.

[anewaname]

Once the first few goblins start burning, there will also be smoke.

[vjek]

I'm planning to build a pump that floods my fort's entrance with magma, but I'm not sure how to make it kill goblins versus just scaring them away.

If goblins are pathing into a fort and magma comes flowing towards them, will they just see it and run away? Do they need to be locked in by bars/grates?

Having had a fair amount of success in this area, I can offer this advice:
- Ultimately, there is a bug in DF (for many years) that breaks pathing when magma flows over pathable tiles.  It's consistent, and reloading fixes it, but while you're testing, this is very important to know.  Why?  Because even if you build the perfect trap, your previous magma filled path will be 'blocked' from the perspective of invaders, and you will never get any more, until you save/reload.  This often gives the impression that something else is wrong, when it's just this.

But, setting that aside, the following designs & features work with great efficacy..
- doors with pressurized magma positioned in such a way that it flows when the door is opened.
- screw pumps behind fortifications, pumping into a hallway level with the output of the pump.
- rooms with automatically/temporary sealed drawbridge-walls, driven by various forms of dwarven logic machines.
- using pressurized magma cisterns above the flooding area.
- using volcanos as infinite magma sources (you can tap a volcano at any Z-Level, 100% safely, using a drawbridge-floor and digging from beneath it, up at an angle, across the entire face of the volcano side)
- pumping out of the top of a volcano, into a hallway, back into the volcano
- using stairs (both up and down to soft-seal a flooding hallway with enough pressure
- you can flow magma off the map by building fortifications on the map edge, underground, in stone.  This means you can build a flowing magma moat that is toggle-able via floodgates, sourced from a volcano.
- you can build a pressure plate, configure it, link it to dwarven logic, and then remove what is beneath it so that it has a draining/empty tile beneath it, so it doesn't ever prevent pathing.
and.. (imo)
- invaders run faster than you think, per tick, after crossing pressure plates.
- some creatures can be submerged in magma and survive
- most creatures do not survive being submerged in magma
- magma will push EVERYTHING through fortifications, if there is enough pressure, in the flow direction. Including, but not limited to, entirely unscathed trolls, beak dogs, and goblins.
- magma mist is extremely effective in destroying invaders, but it often tricky to get it to flow next to their walking path
- it takes a fair amount of time, fully submerged, for some metal and other materials to vaporize, if they're not iron or steel or magma safe.
- dwarves will definitely, without question, run to collect equipment collected in, near, beneath, or as a result of a magma flood, unless you prevent them with burrows, locked doors or something similar.
- the most efficient magma drowning hallways are only one tile tall.

Some funny relevant videos. (unless you're a goblin)

[Boochdizzle]

- using volcanos as infinite magma sources (you can tap a volcano at any Z-Level, 100% safely, using a drawbridge-floor and digging from beneath it, up at an angle, across the entire face of the volcano side)

Can you provide a little more detail about that? I'm not sure I follow what you're describing. Both the way you phrased it, and the wiki language, I'm not really clear where the dwarf if standing, what they're mining, or how the bridge is protecting them.

[Bumber]

Can you provide a little more detail about that?

Dwarf beneath lowered/unretracted bridge breaks physics to mine wall adjacent to bridge on z-level above. Magma flows over bridge, above dwarf's head.

[vjek]

Can you provide a little more detail about that? I'm not sure I follow what you're describing. Both the way you phrased it, and the wiki language, I'm not really clear where the dwarf if standing, what they're mining, or how the bridge is protecting them.

Sure, in addition to what Bumber said.. I'll expand on the wiki language:
" First, channel a hole next to the place you want to breach the volcano, creating a hole and a ramp.
Then, place a one tile magma-safe bridge over the ramp.
Third, forbid the area the bridge is in -- blocking the area off with a forbidden door, or a contraction, depending on how your fortress works.
Then mine the area out from the ramp below. "

So, there are four steps outlined in the wiki, but I'll elaborate a bit more.
Let's say the top of your volcano is at Z-Level 80, and that's your embark level.  Your caverns start at Z-level 60, but you want to make a magma moat at Z-Level 70.
-Dig down 10 levels from the embark level and then dig out a hallway towards the volcano side wall at Z-Level 70.  Dig right up to the obsidian wall of the volcano.  Put a magma safe door on the hallway you can lock, back near the staircase you dug down with.
-Channel one z-level down, in front of the obsidian wall of the volcano, so there is a ramp or ramps below, across the width of the hallway.  Might be one tile, might be 10, up to you.
-On Z-Level 69, that is, one Z-level beneath the Z-level you want the magma to be, where the ramp(s) you just dug is/are, dig another access hallway but this one just has to give your miner access to the up-ramps you just made on Z-Level 69.
-This is what your miner will use to remove the obsidian wall of the volcano on Z-level 70, FROM Z-Level 69, by using the ramp to stand on.
-Build a drawbridge over the channel you just dug on Z-level 70.  Attach it to a lever. flip the lever, so it's down/covering the channel(s). It can either be exactly the size of the channel(s), or bigger, it doesn't matter which.
-Now, lock/forbid the hallway door on Z-level 70.  This area is about to become very unsafe. 
-Finally, designate the obsidian wall of the volcano, on Z-level 70, to be dug.  A miner will walk down to Z-level 69, stand on the ramp on Z-level 69, and mine out the tiles on Z-level 70.  Voila, magma flows on Z-level 70.
Miner walks away and continues their un-vaporized existence.

[Panando]

- using volcanos as infinite magma sources (you can tap a volcano at any Z-Level, 100% safely, using a drawbridge-floor and digging from beneath it, up at an angle, across the entire face of the volcano side)

Can you provide a little more detail about that? I'm not sure I follow what you're describing. Both the way you phrased it, and the wiki language, I'm not really clear where the dwarf if standing, what they're mining, or how the bridge is protecting them.

Here's my attempt.

• A dwarf can stand on a ramp to mine out a tile on the z-level above the ramp, if they couldn't do this ramps wouldn't be very usable when digging upwards.
• When a dwarf is standing on a ramp, they are still in the same z-level as the ramp, in order to be in the z-level above the ramp, they'd have to be standing on a floor or similar thing, but ramps don't generally have floors above them (unlike stairs, where the downstairs is a floor), and if the dwarf was in the z-level above the ramp they'd be floating in the air.
• So the dwarf is in the same z-level as the ramp, but co-locating with the ramp lets them mine out tiles on the z-level above.
• The game is not diligent about checking if there's anything in the z-level above that would block the dwarf, in particular while it checks for "constructions" (floors and such) it doesn't check for "removables" (bridges, hatches, floor bars etc).