Any object that is normally fashioned out of a naturally-occurring ("rough-hewn") wall can also be built on a completely empty tile as a construction. This includes stairs, ramps, fortifications, and new (dwarf-constructed) walls and floors.
Although "built", constructions are not the same as buildings (such as workshops and furniture), instead functioning as inert terrain objects. Once built, you cannot interact with constructions via the or menus (although you can suspend or cancel their construction via these keys).
Constructions behave the same as objects fashioned out of natural rock, although the manner in which they are removed is different and they cannot be smoothed or engraved (even if built out of stone). Stairs and ramps made from natural rock are removed via "Remove Up Stairs/Ramps" (mined out ( - ); and natural floors are removed by digging a channel ( - ). However, all constructed objects of any type are removed via "Remove Construction" ( - ). No specific labor is required to remove a construction, regardless of what labor was required to build it: even children and nobles can and will remove constructions (in fact they do so with glee).- ); non-constructed walls and fortifications must be
Building a construction on a smoothed or additionally engraved floor will remove the smoothing (see removal behavior) and also the engraving. This will give the engraver the usual unhappy thought from art defacement in the case of a masterful engraving.
The most valuable material constructions can be made from, apart from adamantine wafers (worth 1500☼ each), is soap made from the tallow of a [MODVALUE:50] creature such as a dragon, which is worth 1250☼ a bar.
 Strange removal behavior
Once a constructed wall or floor is built, then removed, the remaining floor changes to reflect the base rock of the tile, rather than any stone, ore or gem vein. For instance, if you were to build a wall on a square of a mined-out hematite vein within a basalt rock layer, then remove the wall construction, the floor underneath would become basalt, rather than the original hematite.
More to it, if the floor was smoothed, it will become rough again, and engravings will not return.
One can use this behavior to their advantage in the removal of immovable boulders outside without having to leave smooth stone behind. By building a wall on top of a boulder and then removing the wall, the boulder will be destroyed.
Performing these steps on the bottom layer of ice on a glacier will result in a random type of soil; doing so on above-ground cast obsidian will result in a floor whose material is that of the topmost layer of the biome (usually soil, though likely stone in a mountain biome).
 Priority - LIFO
Construction jobs are filled in order of "Last In, First Out" (aka LIFO), also known as a stack (as opposed to a queue). As such, if you order new construction, workers will attempt to complete those constructions before proceeding to address other, previously designated constructions. Note that if a dwarf has begun to work on a tile, they will not abandon it to work on another (but might to take a break, etc.)
(The trick, therefore, is to designate what you want to be built last first, and designate your high-priority, urgent constructions last. If you wish to make more designations before a priority project is finished, remove those priority designations and then re-designate your highest priority constructions last.)