There are a few different things you might be looking for under the title "Age of Castles." Depending on whether you're interested in a research paper, a book, or a game, here are the most likely matches:
: A classic fantasy-based strategy game where you play as a king (or other characters like a wizard or warrior) building a kingdom, managing taxes, and fighting off enemies like dragons and goblins. Age Of Castles
(2005): This academic paper by Oliver J. T. O'Grady explores how the architecture and geography of castles in the Scottish Highlands reflected social changes and the birth of landed estates. There are a few different things you might