Alternate history is a subgenre of science fiction which explores what would happen if events followed a different course from some key point of divergence. For the vehicle designer, this opens up a rich vein of possibilities for plausible forms that might have been.
The paintings in this post were paperback covers from a series of science fiction novels by Kirk Mitchell. His first in the series, Procurator, began with Pilate pardoning Jesus instead of Barabbus and the Roman army defeating the barbarians. From that beginning, a chain of events allowed Rome to flourish and develop advanced technology.
How would Romans design an airplane? First I wanted to use an unusual configuration that looked somewhat believable. I tried a canard design with a pusher prop, and a variety of design features that suggest the look of Roman galleys.
Many alternate histories are premised on Hitler winning World War II, or the Confederacy winning the American Civil War, or the Spanish Armada conquering England. History would have taken an alternate pathway. New technologies would have been invented, but they would have followed different lines.
From the design perspective, an alternate history premise offers a lot of scope for invention. The author mentioned “sand galleys” and I imagined them as fortified tanks that could cover a variety of terrains.
My beginning point was to fill a few pages of a sketchbook with drawings of actual Roman material culture: the costumes, weapons, vehicles, and architecture, noting their characteristic colors and symbols. Then I tried to extrapolate those features into new technologies. Above are some of the development sketches for the sand galley, an open-topped troop carrier, battle platform, and siege weapon built along the lines of a trireme.
Wikipedia on "Alternate History," link.