Comment by whywouldyoucare

https://en.wikipedia.org/wiki/Monte_Testaccio

It is 35 metres high now, 20,000 or so m^2, after 1500 years, and composed of ancient pottery fragments (the trade went sharply down after the 5th century A.D.)

Caesar reports of a wall more than 20 km long, several meters high, with a canal in front of it, constructed by a legion or two, I don't remember now exactly, in two weeks time. Maybe an exaggeration, but..

And the real question for me, is how they managed to do the logistics, with just the Roman numbers.. So the economical complexity should not be such a surprise, I suppose.