Update: I had to up my rating from 3 to 4 stars after reading a couple of other books about castle history and constructions, including David Macaulay's. This was the book that had the most detail appropriate for my son's age (6.5) and interest, even if it was lacking in some details. It sparked the most discusssion and he has incorporated lots of archaic castle terms into his everyday vocabulary because of it.
Initial review: I'm not generally a fan of the picture/caption books, as I find the informaton they choose to include can be random and spotty. But this book worked well for us as it's not a strict history, so it's okay if the information is spotty (and it is). A few bits of vocabulary about castle architecture and siege-craft were all my 6-year-old son needed. I looked up a few things on the web to supplement (there's no pronunciation guide even in the glossary, so I had to find how to say things like motte and machicolation) or explain vagaries (why are they called hoardings? Are they always wood?) But on the whole, we spent 2 days combing over all the information, with me doing quick drawings to illustrate the concepts. It was fun!