Bridges are extremely useful buildings for crossing dangerous terrain and also for fortress defense. Using them to control fluids can save a ton of mechanisms and time, especially when the fluid in question is free-flowing and not pressurized (by pump, river or hydrostatic anything) and needs a wide tunnel.
 Building Bridges
Bridges can be built (metal, stone or wood. They are first designed by an architect, then require a specialist worker for the material used (e.g. a mason for a stone bridge). The size of the bridge can be altered with while placing it, up to a maximum size of 10 squares in each direction. The bridge must be anchored to a solid surface on at least one edge. Before placing the bridge ensure that the bridge raises in the direction you want it to using or retracts using .-> ) of
Materials: When choosing materials, the order that they are presented on the list determines how the bridge will be labeled, NOT WHAT ORDER YOU PICK THE MATERIALS! The highest one up on the list is the core construction material. This will define the color of the bridge (and possibly how fire resistant it is, although this hasn't been tested extensively). Materials are placed on the list in order of distance, so simply make sure the primary material is the closest or at least closer than any secondary materials you wish to use. You will need the number of tiles divided by four plus one ( Tiles/4+1 ) of material to build the bridge.
Material does not appear to influence dragonfire which will destroy bridges. Some magma-safe materials including iron have proven non-resistant (Needs further testing).
Big bridges can take weeks or even months to complete. You can shorten construction time by moving the materials to the site before starting construction, and by using blocks instead of rocks. While the material-gathering time is the same for rocks vs blocks, the actual construction is three times faster for blocks.
 Raising and Retracting Bridges
If a bridge is set to retract when the lever is pulled, the bridge essentially disappears dropping anything (friend, foe, or object) on the bridge onto whatever is underneath. Clearly this can be used to drop your enemies to rocky/watery/fiery deaths (or anything more imaginative you can think up!).
If a bridge is set to raise when the lever is pulled, the bridge becomes a wall along the edge selected with the keys when placing the bridge. The resulting wall is always one z-level tall, regardless of the length of the bridge. The wall acts as if it was constructed, rendering it invulnerable to building destroyers. Verify The bridge also "moves" to this position very fast, firing anything on the bridge into the air. The key advantage to raising bridges is the creation of a wall when the bridge is raised. This can be used to block fortress entrances/corridors. Using 2 bridges at opposite ends of a corridor creates a very large and simple trap by walling in enemies. Or... Smashing them to tiny bits if placed to raise facing each other, with no space in between. For added effect, place pressure plates on both ends to raise the bridge when stepped on, to fling them, if there is a floor directly above, they will be stunned, (If there is a floor beneath the bridge) and if nobody is on the pressure plate, they have to be lucky to not be smashed on the floor when the bridges come down. (If there is no floor beneath the bridge) they will fall, sometimes into something very, very bad.
Walls cannot be built along map edges. Because bridges can be built along map edges and then raised to act as walls, they can be used to control where enemies spawn on the map.
The lowering of a drawbridge can also be used as a waste disposal for unwanted stones, refuse, goblins (dead or alive), legendary cheese makers and nobles, to name a few. Even fluids get destroyed (this is especially useful considering lack of chasms in the new version). However, lowering a drawbridge onto a sufficiently large creature (such as a forgotten beast) simply destroys the bridge.
Bridges will not operate if any one creature of size 1,200,000 is on them. This weight limit is not cumulative - a bridge will still retract if a hundred goblins are standing on it, but a single rutherer accompanying those goblins will prevent the bridge from operating. Attempting to lower a drawbridge onto such a creature (in order to atom-smash it) will cause the bridge itself to deconstruct.
It is impossible to channel out stone that is directly under a raiseable bridge when its in the raised position. Likewise digging a ramp under a raised bridge will not remove the floor tile.
While bridges do not provide structural support, the game will still allow you to place unsupported constructions adjacent to them which will result in an immediate cave-in once completed, often tossing the unlucky mason off the edge to a horrible death.
Destroying bridges can be hazardous, as dwarves are not as compunctuous as with constructions and diggings to make sure no one is standing on them before destroying them.
A raised bridge cannot be linked to a lever from the inside - the mechanic must be able to stand in the center of the bridge.
SELF CLEANING BRIDGE
Ever have a horde of goblins sitting on your bridge and preventing you from admiring the elegant beauty of your entrance? I have a simple solution for you!!
I know what you may be thinking, "Could my bridge, with a dozen goblins standing on it, be raised?" The answer is yes! A normal bridge can't!. But, with a little careful design and some dwarven ingenuity, you too can make a bridge that will stop almost any foe!
Step 1: Build a pit!
Don't just build any pit; it needs a special shape for the bridge to function properly.
EXAMPLE: x = channeled out section o = solid ground
If this is your only entrance, be careful to leave ramps at the corners so your dwarves can travel through until the bridge is finished.
Step 2: Bridges. Plural.
We will be making many small bridges. Each number designates an individual bridge (FIRE PROOF MATERIALS ARE STRONGLY RECOMMENDED).
< > = bridge lifting direction (01) = bridge "1"
Notice that "11" has no opening direction, because it needs to retract so you can use catapults or ballista to hit pests around the entrance or anything trapped in the middle. In this design the exit bridges are all 3 x 1 in size.
This can be any length or width (up to 20 spaces wide). Only the basic shape is important.
Step 3: Arming the trap!
This is the most simple step, but also probably the longest one.
Link all of individual bridges to ONE lever. Yeah, it takes a bit, but it's worth it.
Step FINAL: Destruction.
Wait for some unsuspecting victims to begin crossing your bridge, and then flip the lever. Getting proper timing down will likely take practice. Sometimes using the repeat order on the "pull lever" command can be very useful too.
Anything small will be tossed up into the air and then fall into your pit. Anything too large for the bridges to lift (titans) will still be trapped and waiting for your archers / siege weapons / other nefarious plans.
there are many useful variations of this design but the key to its overall success is a series of interlocking bridges that are all linked to one lever. the version that was used to SLAY A SPIDER TITAN WITHOUT LOSING ANY DWARVES!(the spider titan was trapped on one of the middle bridges and killed by an untrained crossbowman) is drawn below. (the actual bridge used was 6 times longer than this but this demonstrates the basic principle.) there was also a fortification wall running along one side of the bridge that the crossbowman fired through.
bridges 01 and 07 at the ends are retracting while the rest of the bridges alternate which direction they open. the channeled out area is drawn on the right.
DISCLAIMER: we are not responsible for inept or lazy dwarves that fail to operate the bridge. Standing on the bridge when the lever is pulled may result in injury or death. Many goblins were harmed during the design of this bridge... and some dwarves.
SINGLE LEVER AIRLOCK
Normally, bridges in open states permit movement, while bridges in closed states restrict it. This can be irritating in the design of an airlock system, as such systems usually either require cumbersome logic systems or multiple levers to function correctly. Using floodgates or hatches for these systems may be dangerous, as they are vulnerable to building destroyers. Instead, consider the following design:
The retracting bridge on the higher z-level is used not to permit access across a channel, but instead to block access to a ramp. Thus, a single lever can be attached to both the drawbridge and the retracting bridge, preventing airlock vulnerabilities due to lever timing. With a suitably long distance between the two bridges, the lever can be placed between the two, permitting easy movement of individual dwarves between isolated zones, via lever profiles.
 More useful uses for a retractable bridge
- Remote controlled entrances to your keep. Built at the top of a ramp coming out to the surface, or at the bottom of a very deep hole, these lever-controlled bridges block flying building destroyers and anything else the world throws at you (magma safe material may be advisable for special situations).
- Ocean drains. Dig out ramps leading up to the first level below an ocean. Build a bridge on that level, directly over the ramps (be sure to leave them in place!) and link it to a trigger. Carefully seal off the chamber to make it water tight. Now with the bridge in place, designate ramps up to the ocean adjacent to the bridge. Diggers with access to the level below the bridge can dig those ramps up from the level of the bridge, allowing the ocean to fill the chamber; even with the ramp squares underwater they can still dig them out. And not a drop of water will touch them... provided they clear out before you pull the lever.
- Caravan exits. A bridge to nowhere, built well above ground level at the edge of the map, can sometimes serve as a handy exit for caravans and diplomats when goblins harass. But sometimes it stops working, and I'm not sure why.
- Cave-ins. Since bridges don't support adjoining rock, it is possible to set up a cave-in so that dust can't come up, dwarves can't fall down, and flying creatures can't come up from beneath the cave-in before you set it off.
- Stops on the elevator. Designate a dumpsite or set up a floodgate at the top of a shaft; use multiple remote-controlled bridges to decide on which level the stuff, water, magma etc. gets off. (bonus: use water falling at one end of the bridge to flush stuff off that was dropped onto the other end without the manual labor)
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