It is a rare thing when I find a book really "taking me to school" on a subject, but I definitely had that feeling while reading Martin Brice's A Chronicle History of Forts and Fortresses. This is especially notable as I wasn't exactly coming to the subject blank, having read quite a lot of military history back in highschool, and having plowed through a lot of books dealing with prehistoric hill forts, etc., since. However, this book (while not going into encyclopedic detail for any of the periods it discusses), presented a very informative over-view of the concept and development of fortification throughout human history.
Structurally, the book is set up in six chronological sections: "The Ancient World", "The Medieval World", "The Renaissance World", "The Expanding World", "The Industrial World", and "The Shrinking World", covering everything from the most basic trench-and-wall dirt hill forts to orbiting missile defense systems. Also, aside from the basic text, there are 15 "special features" which look at a specific subjects, from "Tudor Coastal Defense" to "American Frontier Forts", and 28 "box features" which offer looks at specific locations.
The book is richly illustrated, with everything from aerial photography to ancient depictions of sieges, along with a fascinating collection of vintage post-card images of castles from around the world. Everything in this book weaves together in an impressive whole, relating later fortifications back to earlier concepts, and tracking who learned what from whom. As fortification is a "two-way street", there is also a lot of discussion of siege weaponry, from the most basic catapults to heave projectiles over walls, to long-range rocket attacks. The varying designs of fortifications over the ages have been in response to this "dance", with round surfaces coming into vogue to help deflect early canon fire, and short, squat (with massive walls) designs evolving for defense against later, more destructive cannon, etc.
Also interesting is how Brice traces the cultural aspects seeding design issues. In some areas, the object was to obliterate the enemy, leaving just one group in command of scarce regional resources, in some areas, the object was simply to replace one ruling group with another (sometimes leading to palace coups or peasant uprisings if the existing rulers showed weakness), some attackers would completely avoid the use of fire, as they wanted to take the contents of a fort, not destroy it, others would avoid massive siege weapons because they wanted to assume control of the strong point without having to completely rebuild. The roots of WWI are traced back to "courtly conflicts" where prisoners and castles were taken, only to be used to exchange for other prisoners and castles elsewhere, these patterns of formal diplomacy over the years led to the complex inter-twining of pacts and allegiances that sparked that conflict!
I'd love to get into more detail here, but there are literally hundreds of specific locations, battles, kingdoms, castles, wars, construction techniques, etc., all over the world discussed here, and I'd rather not end up generating a long "name check" list in lieu of commentary.
Needless to say, I highly recommend Forts and Fortresses. It is, unfortunately, just out-of-print (hence being in those clearance bins), so you're likely to have to find this via an after-market source. The Amazon new/used guys have "new" copies of it for just under three bucks (plus shipping), and it's certainly worth that (even though that's not much of a "deal" for a book that was initially a very reasonable $9.98 retail). If you have an interest in castles, history, warfare, or archaeology, you'll likely find this a fascinating read.