A fort ship is a very big boat that's built like a fortress. It's designed to protect soldiers and weapons from enemies and attacks. Just like a castle on land, a fort ship protects the people inside with thick walls, towers, and strong doors. The boat is also equipped with cannons and other weapons to defend itself. This type of ship was often used in the past during wars and battles to help countries protect their borders and keep their enemies from invading. Basically, it's like a really big, tough boat that can keep the bad guys out and keep the good guys safe.