(Spoilers to a book from 100+ years ago I guess) Obviously the book is fiction written in the late 19th century, but the beginning of the book when Hank Morgan is transported back to the six century he experiences the extreme punishment for petty crimes and very stringent class divides with an oppressive system of church and kings rights ( along with slavery).
How accurate was this? How did Mark Twain research on this?
Not very, but it's not really supposed to be. The book is a criticism of romanticist views of Chivalry and the high middle ages, not 6th century post-Roman Britain, when it's actually set. It uses Arthur, Camelot, Merlin, etc, as the setting, but this setting itself is anachronistic. What I mean is, works like l'Morte du Arthur (from which Twain takes various parts), were written 600 years after they were actually set, and presented Arthur and his knights in ways that were familiar to 12th and 13th century French and Anglo-Normans, not as historically accurate 5th or 6th century Romano-Britons. So essentially, the book is set in a pseudo-12 or 13th century, that is called the 6th century, just like the original works of Arthurian romance. For example, he wears heavy plate armour, and complains about it, but no one in 6th century Britain had heavy plate armour (or, for that matter, the 12th century).