Megastructures (synonyms: monoliths, great works; example toponyms: The Stone Kings, The Great Gate of Isuun, the Midnight Arcology) are gigantic, constructed buildings, vehicles, monuments, or other elements. They may be great walls, gates that reach to the clouds, massive megapolis arcologies, expansive aqueducts, or Dyson spheres. They may tower above towns or cities below, reaching into the skies or into near orbit. Often, megastructures are used by worldbuilders to inspire a sense of awe and help set a world’s tone. The scale of these elements speaks to the power, ingenuity, strength, or dominance of any civilization or peoples that built them. Megastructures can also be used to showcase important people, concepts, or facets of a world. A giant statue of a historical figure can be used to showcase a portion of fantasy world history while massive industrial machinery could be placed to illustrate a cyberpunk civilizations’ pillaging of the natural environment.

In post-apocalyptic settings, megastructures are often found in ruins and may have been created by some ancient civilization from an era long ago.