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[jonadab]

Ok, so we're all familiar with the standard "main entrance" design, where you have a drawbridge followed by a short bit of hallway, then a trade depot, then traps and probably another drawbridge just in case, then a hallway that goes past barracks and whatnot before reaching the main body of your fort.  You don't want traps before the trade depot, because it would annoy the merchant wagons.

Well, I wanted to be able to protect a caravan of merchants from enemies, without putting my own dwarves at risk.  So I had the main entrance already built, but then I augmented it with a side entrance:  drawbridge, hallway, traps, bottleneck, more traps,  another bottleneck with another drawbridge, and finally it lets out into the main entrance hallway, outside my inner drawbridge.  (In my current fort, it lets out before the trade depot, but it wouldn't need to; that's just how it happened to work out layout-wise.)  The upshot of this is, you can put your military between the goblins and the merchants but still make the goblins go through traps to get at your military.  So far this all seems pretty reasonable.  It *should* be possible to put your military between enemies and merchants but still have traps between your military and the enemy.  And it is.  Good.

So when a siege came, I put up a citizens' alert, then once all my dwarves were indoors, I closed the main entrance.  But I didn't want to stay besieged forever, so I opened the side entrance.  Maybe some of the goblins would get stuck in the cages.  And a couple of them did.  Still reasonable so far.

But then I discovered something interesting, and perhaps a bit unbalanced:  the remaining goblins appeared to be afraid of the traps, so they just sort of lined up behind them -- still inside the outer drawbridge on the side entrance.  Which I then closed, trapping them.  After a while, I got bored watching them not step into the traps, so I closed the inner one too, checked they were all bottled up in that side-entrance hallway, canceled the citizens' alert, opened my main entrance, and proceeded to conduct business as usual, instead of needing to wait for the remaining invaders to step into traps.

The next year, the goblins brought trolls with them, and I repeated the maneuver, trapping every single goblin and every single troll in the hallway full of traps.  This appears to be a very repeatable strategy.

I'm new to the game, so maybe I'm rehashing old ground, but none of the strategy guides I've seen mention it, and it's so powerful you'd think it would be a stock technique.  Thoughts?

Is the line-up-behind-the-traps aspect of the invader AI as much of a game balance issue as I think it is?

[The Bard]

In my experience, they will eventually get their gumption and charge if you wait long enough. Also, if they see a living thing belonging to your fort, they'll attack with a vengeance.

[jonadab]

In my experience, they will eventually get their gumption and charge if you wait long enough.

Not when they're between two closed drawbridges, they won't.  Remember, this is a secondary entrance specifically for invaders.  I can afford to leave it closed until the next invaders show up.  Update: the real issue here is the delay:   lining up behind the cage traps goes on long enough for every single one of the invaders to get past the first drawbridge, before any of them reach the second one.

Also, if they see a living thing belonging to your fort, they'll attack with a vengeance.

Good to know.

[smeeprocket]

that will work for awhile, but when they start coming in greater numbers it might not. I don't know.

You might be better off killing them and getting your soldiers the practice tho.

[jonadab]

when they start coming in greater numbers it might not. I don't know.

Good point.  Is there a limit to how many of them will stand in one place?  I've already noticed that they have no problem crowding two or three of them on the same tile...

You might be better off killing them and getting your soldiers the practice tho.

Also a good point, especially if I want to send them into the caverns later, which I probably do.  I think for this game I will play with the trap-them-in-the-side-entrance strategy so I can evaluate how much impact these issues have in practice.  It may not be optimal strategy, but I want to see how it goes.

[smeeprocket]

You know, you could also turn it into a drowning chamber. Or flood it with magma if your bridges are made of the right stuff.

There's probably a ton of other fun things you could do to those goblins, too.

[Numeroid]

You're using cage traps? You could just let your military train on the captured goblins.

[utunnels]

I have a similar design. But if their caption fell first, the rest will stop.
So every time I lose patience and sent my squads.

Maybe you can fill your corridor with spear trap, wait for all of them to enter, close the door then pull the level.

[smakemupagus]

Yes it's a stock technique.  The oldest and still most important trick in the book i think   Yes traps are a balance issue. 

You can also use "bait" (livestock, for example) and the ability to magically lock and unlock doors to instantly manipulate the pathing of any invaders, quite safely if they lack "building destroyer" ability.  So to give one example, you can trick them into running back or forth through a field of traps or past a tower full of crossbowdwarfs.

[Lemunde]

I have a similar design. But if their caption fell first, the rest will stop.
So every time I lose patience and sent my squads.

Maybe you can fill your corridor with spear trap, wait for all of them to enter, close the door then pull the level.

 I've actually done this. It works pretty well!

[utunnels]

Speaking of traps, are FB/WBs immune to pressure plates? I want to build a trap which links to a bidge, which will trap the target that steps on it.

[Col_Jessep]

Speaking of traps, are FB/WBs immune to pressure plates? I want to build a trap which links to a bidge, which will trap the target that steps on it.

You can use the building destroyer ability of FBs. Make a small water reservoir and link it with a floodgate to the room you want to capture the FB. Then you need a drain with a pressure plate that triggers on water contact.
The FB enters the room, destroys the floodgate, the water floods the drain and triggers the pressure plate which in turn closes the trap room with the FB inside.

[Niveras]

But then I discovered something interesting, and perhaps a bit unbalanced:  the remaining goblins appeared to be afraid of the traps, so they just sort of lined up behind them -- still inside the outer drawbridge on the side entrance.  Which I then closed, trapping them.  After a while, I got bored watching them not step into the traps, so I closed the inner one too, checked they were all bottled up in that side-entrance hallway, canceled the citizens' alert, opened my main entrance, and proceeded to conduct business as usual, instead of needing to wait for the remaining invaders to step into traps.

.

Is the line-up-behind-the-traps aspect of the invader AI as much of a game balance issue as I think it is?

What's actually happening is the leader of a particular squad of goblins is getting caught in the cage, and then the rest his group is just hanging around near him as though he wasn't trapped. This persists until something actually changes: either a target becomes visible that they can attack directly, or enough goblins get trapped or killed that they attempt to flee. You can see the same behavior with animals, although I think in those cases the non-leader animals in the group will eventually run off the map on their own, even if you don't bring in the leader's cage (on the other hand, maybe they stick around and eventually flee or get killed trying to reach the leader's stockpiled cage in deep in your fort).

Though some invaders will not step on traps that they are aware of (AFAIK this occurs most often with elves and humans who may be aware of your trap layouts from merchants or diplomats), I don't believe this applies to goblins who've actually watched a trap trigger on someone else yet.

I don't really consider it an issue. There are plenty of ways in which you can exploit AI behavior in predictable and safe ways, this is just one more. I expect at some point we'll get an invader re-write and perhaps they'll deconstruct or disable known traps (including traps they've observed to fire) or dig new paths around them, at which point players will figure out more robust methods of passive defense, or find new ways to exploit the AI.

[Kuikka]

Are goblins sieges at war with each others? If you manage to capture entire squads, will those fight gobs who arrive with next siege? I have thought this is the case.

[Evrett33]

Marcus Aurelius LP is doing a dwarf fortress LP called Anvilquested where this exact thing happens (ep #19). Marcus builds a side entrance and the goblins rush in and stop right in front of his traps and just stay there even though they could go around.