[KSS42]

Here’s some great videos on this question from YouTuber ToldinStone - Garret Ryan

Author of Naked Statues, Fat Gladiators and War Elephants

Why was Roman concrete forgotten during the Middle Ages?

https://youtu.be/dbvvlFHCNn4

See also his YouTube series: History of Rome in 15 Buildings

The Colosseum: https://youtu.be/m6iHR8zqbiM

[anm89]

Told in Stone is a fantastic channel. Really interesting without ever trying too hard to be entertaining or dumbing it down