It is often said that Castles & Crusades is the Rosetta Stone of Old School Gaming. It certainly is that, but there is a lot more going on here than just that. Castles & Crusades is very much a stripped down version of the basic 3.x SRD. As such there are lot of concepts that are modern including a one-roll mechanic for all sorts of situations. Though if that were all then there would be nothing separating this from say True20 or other "lite" d20 iterations. Castles & Crusades plays like good old fashioned D&D. The aesthetic here is 1st Ed. AD&D, with the simplicity of Basic era D&D. The concept is noble and one we see in many of the retro-clones. But where the clones attempt to use the OGL to make an older version of the rules, Castles & Crusades makes it's own rules and instead goes for the feel or nature of the game. So while you will see Thieve's abilities represented by percentage rolls in Basic Fantasy or OSRIC and as a skill in 3.x in C&C it will be a Dexterity check. Simple, elegant and easy. The Ability check, whether your abilities are Prime or Secondary, are a key element of C&C.

The Castle Keepers Guide is the guide for Castles & Crusades Game Masters. It is a massive book at 291 pages. There are some obvious parallels between this book and the immortal Dungeon Master's Guide, but I am going to focus on this text.
Part 1, The Character largely parallels the Players Handbook with advanced discussions on abilities, classes and races in Chapter 1. Magic is covered in detail in Chapter 2. Equipment is expanded on in Chapter 3 and non-player characters are discussed in Chapter 4.
Chapter 1 does give the CK more options than just what is detailed in the Players book. For example the 4d6 method is discussed among others. If you prefer the newer attribute modifiers; ie the ones from the SRD, 3.x where 18 grants a +4, then those are also discussed and how they might affect the game. Along with that abilities of 20 or greater (godlike abilities) are discussed. For characters, more options are given and experience levels beyond what is listed in the Players Handbook, typically to 24th level. Chapter 2 on Magic is a must read for anyone like me that loves magic using classes. In particular there lots of good bits on spell components and the prices of various items needed to research spells or make scrolls. The effects of holy ground on clerics is very nice to see. Chapter 3 details a number of mundane and exotic items not found in the Players book. Chapter 4 covers NPCs as allies, adversaries or as hired help. Part 2 covers Worlds of Adventure, or how to build your own fantasy game world. Everything from how many moons, to average tempertures by month and zones is covered. Details you might not ever need, but here for your use when you do need them. I rather liked the large portion devoted to urban settings; something I feel gets shorted in fantasy games. Of course dungeons and other underground environments are covered. As well as air and sea adventures. Other sections detail equipment usage, land as treasure (and running this land once you have it) and going to war. Some discussion is had on Monster ecology as well. Trying to make sense of what monsters live in your world and why. The standard monsters from Monsters and Treasure are discussed with an eye to what they are doing in the world; what is their purpose and ecological niche. Chapter 13: Expanding the Genre is actually the first chapter that attracted me to buying this book. On the outset it covers merging different times with your fantasy world. Say adding guns, Gothic Horror or Pulp Adventures. Chapters 14 and 15 details some of the underlying assumptions of the SIEGE Engine rules powering Castles & Crusades. This chapter makes a lot more sense in retrospective of reading Amazing Adventures. Chapter 16 talks a little more about treasure. Chapter 17 about combat.
Chapter 18 adds some secondary Skills to the game. Not needed to play, but certainly will add some more flavor. A Rogue that only steals magical items for example might have a need for Ars Magica. Finally we end with Character Deaths and Fates.

Castles & Crusades is constructed in such a way that most of the information a Castle Keeper needs is in the Player's book. But if they plan on doing anything other than just dungeon crawls then Castle Keepers guide is a must have. Like the Players Handbook the layout and art is fantastic. I also could not help but notice some really nice pieces from Larry Elmore and Peter Bradley. Always a bonus in my book.

If you are a Game Master of any FRPG based on or around the d20 SRD then I would highly recommend this book. The advice is solid and the mechanics are so easy to translate that it hardly matters what game you are running, it will work with this.