OH My!! I read this book and its successors when I was a kid. So did my dad. Ruth Stiles Garnett is one of the best children's book writers EVER!!
considering it was published, oh about 20 YEARS before three Mile island or Chernobyl, it is pretty accurate to what would really happen.
Both an Alternative history like Piper's Indo-European Volkswanderung Lord Kalvan of Otherwhen, and a subtle joke on the reader if he is not knowledgable of the Napoleonic Period. Look up Arthur Wellsely!
Looking back on this magnificent book, It makes me wonder if this is where George Lucas got the idea for the Ewoks. I love anything by H. Beam Piper