Fortification

Fortification is a construction that allows the passage of projectiles (in and out) and liquids, mist, steam, webs, fire, miasma and smoke, but in theory 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.

Fortifications can be built one of two ways. Firstly, they can be carved from constructed walls or smoothed natural walls by using Carve Fortifications designation. The second way is to build them.

Constructed fortifications are open on top - they do not provide a floor for the tile above. This lets water, items and some critters in, until and unless a floor or something else is built one Z-level above, just like over any other empty space. Fortifications carved* out of a wall (whether natural or constructed) are covered - they retain most properties of the original wall, including a floor one level above.

[Bug] 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.