This book may have been intended for young people when it was first published, but that was a hundred years ago. There are a lot of relatively unexplained references to Scottish history – the Jacobites, the Covenanters, and so on – that make the basic plot hard to follow without lots of background information (which we provide in the "Detailed Summaries" of Chapters 5 and 9).
Making the novel even more challenging is the frequent Scots in the dialogue. Before reading Kidnapped, we had never heard of words like clachan (hamlet, village) or gleg (fast). We could figure out some of these from the context (most notably, "wheesht," which appears to mean "shut up"), but for others, we had to head to the good old Free Online Dictionary. But the fast pace of the plot and the suspense of Davie's adventures makes it totally worth the work. The language may be tough, but the setting is exciting, and Davie is easy to identify with. So overall, we're giving this one a four out of ten for difficulty.