My setting includes a space faring civilization which builds massive terrestrial ziggurat-esque megastructures as one of their main identifying features. They vary in size but even the smallest ones are thousands of feet across and they function mostly as self contained cities.
This civilization was started by colonists who migrated to a distant planet. The planet, while habitable, has a layer of mildly toxic topsoil which is blown about by the wind and can cause chronic health issues via long term exposure. The colonists found that much of this toxic dust could be avoided by gaining a little elevation so they built most of their settlements on flat topped hills and mesas. However the planet is less tectonically dynamic than Earth and much of its land is relatively flat greatly reducing the features desirable for settlements.
When the most of the good hills and mesas had been developed but the population continued growing the decedents of the original colonists resorted to building their own mesas out of quarried stone or concrete. Once the top of these artificial mesas filled up they'd build platforms over the existing buildings (with large galleries to let light and air through) and then build on those. This process would continue until multiple successively smaller layers were built up creating a ziggurat style structure. These megastructures would eventually become the main urban centers for this civilization and when they began to colonize additional worlds they would continue building them on those planets as well.
Now the problem is these structures are huge and require an absolutely massive amount of material and tons of heavy machinery to build. Additionally much of this material is just filler and not used create any usable space. Given the resource requirements what would one, justify building these megastructures as opposed to a bunch of cheaper skyscrapers on elevated pillars, and two, be a compelling enough reason to continue building them even on new worlds where the toxic dust is not an issue?