Reflecting on Toady thinking about invasions and nerfing cage traps, obviously without making defenses useless.
I think a solution is be to add a new kind of soldier - an engineer soldier. In armies engineers build/repair bridges/boats/walls/trenches and remove mines to support the actual fighting troops.
They'd have a title so the player can tell who they are - "goblin recruit military engineers" or whatever. I imagine an army having 1 of these/30 regular soldiers.
I propose to make a skill that allow an invader engineer-soldier to:
Find traps in advance
Disable found traps
Open a cage to release the prisoner
Free themselves from a cage
Somehow neutralize drawbridges*, locked doors and hatch covers.
Maybe more things?
Skill level decides time to complete actions plus success rate - a low skilled engineer taking maybe so long to do stuff that he becomes redundant, like FINALLY getting a guy out of a cage when everybody else is already dead. A lowly skilled engineer should also could go "all clear, proceed" and then walk into a missed trap and get speared to the amusement of the player.
Invasions can be made harder/easier by raising/decreasing number of engineers plus by their skill.
It's a knob that can be turned all the way down to hardly matters - if an engineer has say 50/50 fail rate and he's working on a hallway with 10 spear traps, he won't last long! And it can be turned all the way up with a bunch masterengineers neutralizing traps. And it can be everywhere in between.
I guess the first invasion should just have a lone crapppy engineer destined to end up in a trap he messes up trying to remove, so the player gets aware of them being a thing. And afterwards invaders bring more and more of them so it gets harder later on.
Invader behaviour should be changed with invaders sometimes being wary to rush into non-checked areas. The enginners would have no use if the gobs never waited for them
Invaders should neither always wait for an engineer to work on every single square, nor should they just always barge ahead.
It could maybe be simple with invaders considering areas around where people ran into dangerous, but by default they run ahead? So if they meet traps they stop and work on them or path again but an area 5-10 squares from the trap is considered danngerous?
Maybe invaders should always distinguish between "be carefull" areas (maybe around doors and drawbridges inside the fort?) and "safe" areas (outside, featureless rooms) - so this creates a tactical possibility of the player trying to put traps where the gobs don't expect them to be.
Anyway it's another knob to turn up and down - how much do invaders wait for for engineers to clear areas before rushing in. I guess if their engineers died, they just go for it and rush ahead, after all, they're there to wreck shit.
Consequences:
Now we've got a fun tactical situation - with the player wanting to kill specifically the engineers so they can't remove traps. I presume archers in towers would be useful to pick off engineers protected by meleers.
if an enginners stays alone behind the army to free a bunch of goblins from cages, maybe would make the player build some "secret" exits to alllow surprise sorties to get at unprotected engineers.
It would just be great to create a sort of arms race with players having to get more sophisticated to stop invaders. It would make it more attractive to build some more sophisticated defensed, that to me would feel more like real fort-stuff instead of just a trap hallway.
*IDK what they can do about drawbridges? Realistically you'd have to first install a thing to prevent it from crashing down, like stick some logs to it to force it to remain open, and then afterwards use a battering ram to break through it. Then invaders would bring some logs and they could then use logs both to stop a bridge from closing plus to batter it. Then it becomes a tactical thing to get their logs
I guess though drawbridges could be left for a while as un-nerfed and players can then choose to use them or not.