The fort you can see today was built about 500 years ago by Portuguese sailors who came to trade. Its thick golden walls and towers look out over the calm blue sea.
When archaeologists (history detectives) dug down beneath the fort, they found something incredible: the remains of seven different towns, one on top of another. The oldest is around 4,000 years old, from a trading land called Dilmun. People have lived on this exact spot for longer than almost anywhere on Earth.
Dilmun was famous all over the ancient world as a place of fresh water and busy markets, where boats stopped to swap goods on their way between faraway lands. Standing at the fort, you can imagine traders arriving with dates, pearls and copper.
Because it tells the story of so many people across so many years, Qal'at al-Bahrain is a UNESCO World Heritage Site, a place looked after for the whole world.