Fortresses

CITADELS

Strongholds often built on high ground to protect cities from invaders.

The clue "Fortresses" directly hints at the answer "CITADELS," as citadels are fortified structures often used for defense. The word "citadel" itself is synonymous with a stronghold or fortress, making this a straightforward definition-based clue. There’s no hidden wordplay or pun here—it’s simply a matter of knowing that "citadels" are a type of fortress.