[@Raineh Daze]

I don't think an entire multi-level fortress building constitutes as something that is part of a treasury, that would likely fall under a kingdom at that point, I think a vehicle like his aircraft is as complex as it gets. 

That aside, I would be 100% for Gilgamesh adding onto the hanging gardens with some fortifications only a king of uruk could make. (because why build a fortress by yourself instead of building ontop of the immense base that is the hanging gardens)

Then *maybe* Semiramis would let Gilgamesh inside the hanging gardens without a fight.