I will admit that my main fascination with this book stemmed from the various worlds the characters travel through. The Paul Jonas mystery gets deeper. His adventures in a Neanderthal world were enough to get me to start reading Clan of the Cave Bear
(unfortunately I never managed to force my way through more than a handful of chapters in that book). Likewise, Jonas' adventures on Mars led me to read A Princess of Mars
(I was much more pleased with that book).
My memory isn't what it used to be, so I can't be sure, but I believe
there is a replay attack described in this book. While the attack is described accurately -- and I'm sure many fans thought the attack was very clever -- there were solutions to the problem when this book was published, and I had a hard time believing that the Grail Brotherhood would have fallen for it.