What do we call the strong central tower in many castles?

The story behind the answer

It’s the castle’s main stronghold—the tallest, thickest-walled tower designed as the lord’s residence and the final refuge during a siege. Positioned at the heart or atop a motte, it offered commanding views, secure storage, and a defensible last stand, which smaller wall towers could not match.

Emerging with Norman motte-and-bailey castles, it was often called a “donjon” in French. Early versions were square, but later rounded or polygonal plans reduced blind spots and resisted undermining. Don’t confuse it with a turret (a small projecting tower), a bailey (the enclosed courtyard), or a gatehouse (the fortified entrance). As much a symbol of authority as a military hub, it anchored medieval power—stone made manifest.