Let's face it. The Tough-O-Meter goes up a bit every time we move back a century. It almost has to: in older books, the language is less familiar, the prose is more complicated, and the stories reflect increasingly unfamiliar societies. Moll Flanders was published in 1722, so that's almost 300 years of toughness to catch up on. Who says stuff like "thither" anymore?
To make it even trickier, the book isn't divided into chapters. There are no easy places to stop reading so you can go get a snack. Oh, and most characters can't be identified by name. (For more on this, see "Tools of Characterization: Names.") The combination of eighteenth-century language and eighteenth-century organization lands this book on the tougher end of the tough-o-meter.