There's a castle/medieval themed book in the Magic Tree House series: The Knight at Dawn. There's a Magic Tree House Research Guide book to supplement it: Magic Tree HOuse Research Guide: Knights and Castles. A book to show the kids, not having them read, is Castle, by David Macaulay. This guy writes/illustrates books about how famous architectural structures were built and gives detailed floor plans within his book, and they're directed at children. Hard to read for them, but the pictures and floor plans are interesting and educational. King Midas' Touch, the fairytale, is castle-related, as is Rumpelstiltskin. Good luck!
Do an archive search... several of us have used this theme in the past...
A few ideas I can remember:
A Royal Welcome
Incentive dollars... Castle Cash
Knightly Reading Log
Catch a Dragon by the Tale... relate to books
Apprentices of the Day
Newsletter... Castle Capers
Quest for the Jewels... marble jar incentive