Impregnable underground defenses

dwarf-fortress

Over a number of games, I've never had anything worse than goblins attack the surface entrance to my fort. These are easily dealt with by a few doors and long lines of traps. I almost feel sorry for them.

However, I have had rather more…Fun creatures attack from below – specifically Forgotten Beasts from the caverns. Since these guys are building destroyers, my measly stone doors barely slowed them down.

How can I build a defense that will keep monsters this powerful at bay, or better yet, destroy (capture?) them outright? Are there different tactics available at a cavern entrance vs. a surface entrance?

I prefer constructed defenses over armies, but I realize that for some beasties, the only answer may be a platoon of well-equipped veterans.

Best Answer

A raised drawbridge or retracted bridge over a moat will stop all uninvited guests that can't fly. As you have noticed they are building destroyers so the drawbridge can be destroyed by a Forgotten Beast and it is no better than a retracted bridge to stop flying FBs.

A retracting bridge has the advantage of allowing you to make a manual trap if you time the closing of the bridge properly. I suggest you use a long bridge that leads to a long fall on spikes traps. As soon as you get the message about the uninvited guest, send one of your dwarf in the room where the lever to access the bridge is located (lock him there). It takes some time for the bridge to retract: you will need some testing to determine the optimal time to pull the lever (use pause then . to advance the game slowly). Long bridges are advised to let your dwarf some time to pull the lever -- dwarven stupidity should never be underestimated.

FBs don't activate traps but the spikes in the bottom of the pit don't need activation: they are always on. I'm not sure about the optimal fall distance but a fall of 10 z-level on steel spikes made short work of my last uninvited guest.

Flying FBs are another matter I suppose. I haven't played long enough to get flying Forgotten Beasts but I'm sure they exist and you will have to either wall the cavern entrance (b C constructions can not be demolished by invaders) or confront them with your army.

Related Solutions

Will fish populate artificial underground canal

Yes, water creatures can swim from natural rivers into any connected bodies of water including those dug out by your dwarves.
I believe bars, grates and fortifications all let water flow but keep creatures out, including vermin at present*. The idea for allowing vermin/small creatures to pass through bars (but not grates) has been floated in the past, but to my knowledge this has never been implemented.

Also note this bug: dwarves and creatures can sometimes be pushed through grates/bars/fortifications by water flow, though in my experience this is pretty rare.

*EDIT: @Daenthy correctly notes that vermin can spawn spontaneously in habitats that support them, so vermin may still turn up in your "secure" water sources.

Dwarves stuck underground

There are two simple ways. One is to dig an upward staircase just below the surface and a downward staircase on the surface tile above.

Surface ---\---
Layer 1 ___X###

Where -- is the ground, X is the upward staircase, \ is the downward staircase, # is filled and _ is your tunnel. You need to dig the upward staircase first, or just designate both at the same time and your dwarves will dig the proper one first.

The second way is easier. Just dig out a ramp on the level below the ground. Ramps automatically clear the floor from the tile above them. The only tricky bit is that ramps need to have solid ground under one of their sides, like so

Surface ----- ----
Layer 1 _____/####

Where the symbols have similar meanings as before with / as the ramp. If you don't have that filled tile on one side of the ramp then dwarves cannot exit the top of the ramp on that side. So you need to remember to have a filled tile with open ground above it on at least one of the adjacent tiles to the ramp. Just to be safe make it two or three.

On a related note, channeling should leave a ramp behind, so if your miners are stuck it is probably because you either removed the ramp, or dug out under all the adjacent tiles. This is a common mistake to make because the exact nature of ramps is a touch complicated. The wiki has a pretty good explanation with much better pictures than I do. It also has some explanation about stairs with a good diagram of how each tile is divided into floor and wall sections.