Joe, the way I learned it in 9th grade Latin class was that the word for a military fort was castra, which is the plural form. The reason was that a Roman fort was built with four towers at the corners. Each tower was called a castrum, so the fort itself was called the castra, or "towers." I believe our word "castle" comes from castellum, meaning "little tower." Of course, my memory might be incorrect; after all, that was XXXIX years ago...

Aloha,
Mark