Opening the book…
Grand invented worlds often collapse under a simple question: what does anyone here eat, and who paid for it. A floating city, a desert empire, an eternal war, each implies a mountain of unglamorous logistics, and a world that ignores them feels weightless, however gorgeous its towers. Material logic, where food comes from, how goods and armies move, who is rich and why, is the skeleton that holds the scenery up. Readers rarely notice good logistics, but they feel bad ones as a vague unreality, a nagging sense that this economy could not actually function, that these people are being fed by the author rather than by any visible field or fleet.
Ask the peasant questions of every impressive thing you build: what feeds this city, what does it export to pay for what it lacks, how long does a journey take and what does it cost, who does the invisible labor. Let the answers shape the plot, since trade routes make natural roads for stories and scarcity makes natural conflict. Follow money and food into your scenes as texture: the price of bread, the guarded caravan, the port thick with the wrong flags. You need not calculate harvests to the bushel, but you should know, roughly, why this place is not starving, and let that knowledge keep the world's feet on the ground.
Some modes, myth, fairy tale, surreal or dream fiction, deliberately float free of logistics, and demanding a supply chain from them would be a category error. Comedy and fable get similar license. The rule bites hardest in the grounded, realist end of the fantastic, where the whole appeal is that this improbable world could actually work.