From the World Heritage inscription:
Qal’at al-Bahrain is a typical tell – an artificial mound created by many successive layers of human occupation. The strata of the 300 × 600 m tell testify to continuous human presence from about 2300 BC to the 16th century AD. About 25% of the site has been excavated, revealing structures of different types: residential, public, commercial, religious and military. They testify to the importance of the site, a trading port, over the centuries. On the top of the 12 m mound there is the impressive Portuguese fort, which gave the whole site its name, qal’a (fort). The site was the capital of the Dilmun, one of the most important ancient civilizations of the region. It contains the richest remains inventoried of this civilization, which was hitherto only known from written Sumerian references.
Bahrain isn’t a very big country. You could probably drive from end-to-end the long way in an hour. Despite its size, it has been an historically significant port. The image above is of the old ruins of the ancient harbor. Just on the other side of these ruins, not in camera, are the ruins of an old Portuguese fort which was on the site. There is also a museum on the site which is the best cultural center I’ve seen in the Gulf.