A bridge defaults to a single tile in size. You'll need to change the size to suit your needs with the k,h,u,m keys; the bridge's dimensions can be set anywhere up to a square 10 tile on a side. A bridge will require both architecture and masonry (or carpentry or metalsmithing if wood or metal is used) to complete. The material required for a bridge is less than the material required for building an equivalent area of floor tiles (1 + area/4, rounded down), and when a bridge is deconstructed the materials are deposited on the ground next to where the bridge stood rather than falling down into the open space it spanned.
Unlike raising bridges, which require an area of solid ground, retractable bridges can be built using existing bridges as their sole means of support. However, the same cannot be said of other structures such as floors. The game will allow the structure to be placed, but as soon as it is built, a cave-in occurs and the unfortunate dwarf constructing it falls to his death (bug 000436).
Interestingly, building a bridge over a constructed floor will allow dwarves to deconstruct the floor through the bridge without disrupting the path across said floor (especially if the floor is being used to bridge an area). Conveniently, the materials used to construct the floor will be left on top of the bridge, rather than being dropped down below it.
If a bridge is built outside over a channel, anything under the bridge counts as inside for the purpose of preventing cave adaptation (but not for the "stay inside" order), regardless of whether or not the bridge is actually covering the area.
A drawbridge can be connected to a lever or pressure plate. When the lever is pulled, after a delay of 100 steps the bridge will raise in the direction specified at construction. The direction that a bridge raises can be slightly confusing: if a drawbridge raises to the left, then it forms a wall on the left side when raised. A drawbridge that raises requires a floor underneath the side towards which it will raise (on the tiles where the "wall" will form when raised); remember to take this into account when digging.
A drawbridge, when raised will crush anything standing on the tile that becomes a wall, and when lowered it will crush anything that is standing where the drawbridge exists when lowered. This has earned it the name dwarven atom-smasher. Some players may consider the crushing power of the drawbridge to be an exploit — the only units that survive the smash are those with a [SIZE] greater than 10 ( megabeasts, demons, and the wagon ). - rather than being crushed, they will cause the bridge to instantly deconstruct.
Any creature standing on a drawbridge when it raises will be flung a between 1 and 11Verify units to the sides. This applies to all unitsVerify, including flying creatures who happen to be on the same tile as part of the bridge at the time. The creatures will usually come out from the fling stunned but otherwise uninjured, and flying creatures may drop out of the sky.Verify (To see what happens during a fling, view the Stone Fling DFMA movie.)
Any item sitting on the drawbridge has a 1/4 chance of being flung. They can be flung anywhere from zero to 11 Z levels upward and up to 11 units in any other direction.
Any creature standing on a retracting bridge when it retracts or extends will be flung . This applies to all unitsVerify, including flying creatures who happen to be on the same tile as part of the bridge at the time. The creatures will usually come out from the fling stunned but otherwise uninjured, and flying creatures may drop out of the sky.Verify If a creature with a [SIZE] greater than 10 ( megabeasts, demons, and the wagon ) - is standing on the bridge when you attempt to retract it, it will simply fail to operate.
Any item sitting on the retracting bridge has a 1/4 chance of being flung. They can be flung anywhere from zero to 3 Z levels upward and up to 3 units in to the sides.