Rome's downfall was already set in stone before it was built.
You see, while construction began, they didn't have the necessary workforce to help during this project. But luck was on their side, and the Italian mafia offered their services for a big sum of money.
Thus the saying "Rome was built in three days" came from. But with every lucky convenience, misfortune came after. It turns out, after closer inspection, the whole town was built with breadsticks, paint, and paper mache, causing the deaths of thousands and a successful business profit for the mob.
(I got another one while writing the above one: Rome fell because THEY WEREN'T THERE AT ELF PRACTICE!)