Though there are some famously large geoglyphs, most of the ones in this article were relatively small (10-20ft across).

And the desert is also just ... Huge. As a challenge/demonstration, go to Google Earth/Google Maps, go to the first location that pops up for the Nazca Lines and see how long it takes you to find them/how many you can find.

"never attribute to Halon that which is adequately explained by a careless typo"

Not a fortification as such but you may be interested in Medieval/Early Modern armored ships. definitely some interesting designs that are often glossed over