Skirmisher Publishing has released Castle Builder Volume 5: Keeps. Go build yourself a castle.
From the release:
Castle Builder Volume 5: Keeps is the fifth entry in a 10-part series designed to help Game Masters and players alike plan their ideal fortifications in a step-by-step format from the ground up. This installment looks at the border structure and way station that serves as the first-line defenses of a realm.
* An introduction to the idea of keeps, why people build them, and how they are perceived;
* Various functions of such structures;
* How to consider the most suitable uses for keeps in your game;
* Typical locations for keeps;
* Floorplans and description for an Anatolian Square Keep, a complete, four-level sample fortification designed to house a company of infantrymen.