40d:One-way

Currently^{v0.28.181.40d}, due to what seems to be a minor bug rather than a feature, it's not only possible but quite easy to create one-way gates with ramps - and with those, one-way halls, stairs, and any other restricted path you can create from one controlled access point to another.

The set-up has only a few requirements:

• 1) A closed route, with no "normal" entrances or exits.
• 2) (At least) 2 z-levels over 3 tiles in a line, enough for an up-ramp, a supporting wall-top, and a down-ramp on the other side, forming a one-way gate.
• 3) Two such gates, one at each end of the enclosed route to ensure the restricted one-way traffic flow.
• (For newer players, make sure you understand how ramps work in DF - they're not purely intuitive!)

Lack of the first would allow any dwarf to wander in and go where they want from the "leak". Having only 1 gate at one end would, again, allow a counter-directional leak at the far end.

(If the route has nothing to cause dwarves to seek a job or path there, such as a long empty hallway from one area to another, that last may not be a problem since they cannot exit the far end and so have no (good) reason to ever enter. You could still send military in if you had reason to, but tantruming dwarfs or other creatures could as well. Your own tame animals could also wander as they please in such a setup, unless restricted by pet-locked doors or hatches.)

Basically, the trick is that when you build a ramp under a constructed floor (not a natural floor!), and then remove that floor, the game fails to recognize that as a down-ramp but still sees it as an up-ramp, and so only the "up" half of its path is recognized. Since neither the game nor your dwarfs can see or find a down-path, they only have one way off the upper level - one way.

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Step by step

This description will be completely constructed - the first two steps could be a natural up slope to a natural floor and wall below it, but the next two must be constructed. Once you understand how it works, variations on design are certainly possible.

• 1) Build a wall (b, C, w). This can be 1 tile or many more than 1, but the gain in z-levels is necessary.

  side view:  ___■___

   top view:     O   (single wall)

        z+1:     +   (single wall-top)

• 2) Build a ramp next to the wall (b, C, r). This will be your "down ramp" to define your direction of traffic flow.

        <-- traffic flow <--

  side view:  __/■___

   top view:     ▲O   (up slope & single wall)

        z+1:     ▼+   (down slope & single wall-top) 

• 3) Construct a floor (b, C, f) extending out over space above where your "not-a-down ramp" will be constructed. (This will be your "up-ramp" to determine direction of traffic flow.)

        <-- traffic flow <--

                  _
  side view:  __/■___

   top view:     ▲O   (up slope & single wall)

        z+1:     ▼++  (down slope, single wall-top and constructed floor) 

• 4) Construct your "up ramp" under the floor, (as step 2).

        <-- traffic flow <--

                  _
  side view:  __/■\__

   top view:     ▲O▲  (up slope, single wall, 2nd up slope)

        z+1:     ▼++  (down slope, single wall-top and constructed floor) 

• 5) Deconstruct the floor (d, n).

        <-- traffic flow <--
 
                  ..
  side view:  __/■\__

   top view:     ▲O▲  (up slope, single wall, 2nd up slope)

        z+1:     ▼+   (down slope, single wall-top...and no down slope!) 

Success!

It's recommended you play with this just a bit first in a sandbox mode, to understand both the concept and how it works in practice. By adding this "one-way entrance" before a set of stairs or normal ramps, you can create one-way traffic flows for any situation - perhaps even caravan wagons. ^[Verify] The initial down-ramps do not have to be connected (or even present!) - it's the "not-a-down-ramp" that's the key, so using stairs in place of the fully-functional ramp would be fine. So long as there is at least one in-gate and one out-gate, more complex configurations can be created, forming Y's, T's or X's, whatever traffic flow you need.

Video Example

A veteran player has posted a video demonstration here: One-way passage example

It is purely a demo: from an embark situation, he builds his walls and ramps (designating them at the same time) - if he did not build the 2nd, dwarves might wander in from the other side. He then* designates his constructed floors, and then his 2nd ramps.

^{(* This author is unclear why the "Restricted" traffic zone was placed - in tests, it did not seem necessary.)}

Once the ramps are built, he designates deconstruction of the two floors and channels out the sides, creating a restricted path between the two gates. Once the floors are deconstructed, we can look at the space above where we know the 2nd ramps are: they read as "Empty Space" from this upper level, but dropping down to ground level, there they are: "Bauxite Block Upward Slope"! Each wall now has two up, but only one "down" ramp, and a closed path between them.

He then designates a garbage dump ( i, designate area, g ), remembers to turn on "Dwarves Gather Refuse From Outside" ( o, r, o ), and marks all items for dumping ( d, b, d, designate area ).

And (after a tense moment as the dorfs think about it)... the show begins.*

^{(* Complete with a couple headfakes by true dorfs who take the long way around, just because.)}

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See also:

• Mega-project
• path