I wanted to know more about the design and features of castles through the medieval period, and this book seemed to fit the bill. The good points: easy to ready, well written, takes a reader on a logical journey through castle development and design sources (e.g. from the Middle East) and how these ideas were incorporated into fortifications. Castles have evolved remarkably over time.
The only negative points about this book are: it throws in ideas such as brick course, timber strengthening and so on without actually explaining what they are and how they were constructed (and their specific benefits). Second niggle: a little light on actual full castle plans/schematics.
A very good book in all.