The Pastoureaux (or Shepherds Movement) of France are part of the plot, and the Children's Crusade is also mentioned. A dark but fascinating part of history I wasn't aware of prior to reading this book and was inspired to research afterwards.
The loss of King Johns treasure at the Wash in 1216 is also a large part of the plot, as rumours and myths surrounding the treasure are rife and some characters devote their lives to searching to uncovering the mystery.
Overall, another satisfying medieval mystery. I thought I was close to the end of the series, but just learned that there are another 8 in the series, so I'm really only half way.
My rating = ****