Some of the aquaducts the Romans built are still standing... amazing structures.
One wonders how much engineering knowledge and know-how was destroyed when the barbarians overran and sacked Rome, destroying libraries, books, etc.,
Dark Ages, indeed...
I would think to destroy the engineering you would have to kill all of the engineers. Some things are handed down through the work. The muslims destroyed the library at alexandria supposedly the largest compository of documents and books in the world at that time.
The vatican has a lot of roman and church stuff all the way back to 2nd century AD
Like most of us here, I also appreciate barbarians, because, y'know, they're a big part of my ancestry. :^ ) I'm reminded once again of the finale of Herodotus' Historia:
Cyrus, who did not greatly esteem the counsel, told them, -- "they might do so, if they liked -- but he warned them not to expect in that case to continue rulers, but to prepare for being ruled by others -- soft countries gave birth to soft men -- there was no region which produced very delightful fruits, and at the same time men of a warlike spirit." So the Persians departed with altered minds, confessing that Cyrus was wiser than they; and chose rather to dwell in a churlish land, and exercise lordship, than to cultivate plains, and be the slaves of others.