Fortifications are a special type of wall. They allow the passage of projectiles [in and out] and liquids, mist, steam and smoke, but not creatures, making them an important part in a fortress's defense. An archer must have a skill level of accomplished or higher to shoot through fortifications from a distance; otherwise they must stand directly next to the fortification to shoot through it. Creatures with a [FLIER] tag can of course fly over and avoid these walls. One can build floor tiles on the Z-level above, to make a roof against flyers.
Fortifications allow liquids to pass through in any direction except vertically -- they are open at the top, allowing liquids to be poured in from above, but they implicitly include a floor (even when constructed over empty space) so liquids will not fall out their bottom.
Note that goblin elite bowmen and crossbowmen have sufficient skill to ignore the distance from your fortifications; they will shoot right through every time and (likely) decimate your forces. This can be avoided without compromising the usefulness of a fortification network by constructing a thin 1-tile wide raising bridge just outside the fortification. Attach each bridge to a lever and pull the lever if an elite goblin poses a threat. The blocking bridge will cut off line of sight and prevent bolts from penetrating your defenses. This will likely backfire if used on ground level, however, due to the presence of building destroyers during sieges.
Beware that fully submerged (i.e. 7/7 depth) fortifications will not block the passage of creatures that swim in water (or magma) - wall grates and vertical bars work, but they are vulnerable to building destroyers. A partially-submerged fortification (even with 6/7 depth) completely blocks the passage of creatures, despite the fortification being invisible. If the water is flowing though, it can push creatures through fortifications, even if they are not fully submerged (that is at least true at 2-3/7 depth).
Curiously, a fortification carved into a tile at the very edge of the map will allow water or magma to drain through it and off of the map.
In adventurer mode you can throw any item (such as bows, arrows, axes, hammers, shields, body armour, severed limbs, corpses, etc.) through the wall, but you cannot climb through them.
 Building Fortifications
Fortifications can be built one of two ways. Firstly, they can be carved from constructed walls or smoothed natural walls by pressing esignation and then C rve Fortifications. The second way is through the construction menu: First press , then followed by ortifications. As with most buildings, this will require one unit of wood, stone, metal, or glass. Note that constructed fortifications do not have walkable floors above them, while carved fortifications do.
Carved fortifications must be carved by a dwarf with the Stone Detailing labor enabled. Constructed fortifications must be built by a dwarf with a corresponding labor to the material used.
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