Two summers? Either you've got some way to keep sand sculptures up for two years, or you've spent the first summer planning, designing and testing, and the second building, or something else I'm not seeing here.
Well done! This is very impressive, and even more so when you realise it's actual size.
Your right that sand sculptures don't usually last very long. I use a sandy loam which vary's from traditional beach sand in that it forms mud, which when dry is relatively strong. For example the city was strong enough that I could stand on top of it without it breaking.
It took me one summer just build the stepped mass of the city, which I then covered with a tarp to protect from the winter. And a second summer to do all of the detail work.