I have a lot of questions about Roman Britain. I hope it's not against the rules to post multiple questions, as they're largely grouped around a single theme.

• At any given time, what number of military troops were typically stationed in Roman Britain? What was considered the minimum number to ensure Roman authority?

• What was the ratio of Roman "immigrants" to Britain compared to natives?

• What proportion of the general population could speak and understand Latin?

• What kind of infrastructure did the Romans build, in addition the famous roads and Hadrian's wall? For example, did any town of Roman Britain have sewers like Rome did? I know that London didn't have a sewerage system until the 19th century, so I'm guessing not, and yet they did build baths.

• What were the major products of Roman Britain? I'm aware of wool and tin. But I've also read that there were silver and gold mines in Britain during this period, which surprised me. What kind of quantity of silver and gold did these mines produce? When were the deposits exhausted? Did the Romans discover/mine coal?

• Where did Roman Britain trade with directly? Did ships just shuttle between Britain and Gaul, or would British ships make their way to the Mediterranean?

• What global trade goods made their way to Britain from the wider empire? Would anyone in Britain have their hands on goods like silk and cinnamon, for example?

• Would there have been any place in Roman Britain that would have had a substantial library? I'm thinking in particular of Greek and Roman philosophy/mathematics, but also Roman technology/engineering knowledge.

Thank you for your patience, hopefully someone might be able to answer some of these! If you're wondering, the motivation behind all these questions is that I'm interested in alternate history and specifically in what would have had to occur in order for Roman culture to continue in Britain post-410 AD.