Qal’at al-Bahrain comprises two fortifications. The earlier of the two and the oldest in Bahrain is an 11th C Islamic fort. A 16th C Portuguese fort is located on the adjacent hill. The ramparts, three surviving bastions and the remains of a fourth are surrounded by a dry moat.

Archaeologists have found on the 40-acre site evidence-of seven stratified layers of civilization which range from the third millennium B.C. to the 16th century A.D. The Islamic fort, which may have been built by the Carmathians, is square with towers at each comer. The interior is planned around a paved courtyard. The walls were faced with good quality stone, much of which was later used in the construction of the Portuguese fort.

Fragments from a Grecian-design ceramic bowl were shown to belong to the period around 100 A.D., and predating the foundations of the fort. Coins and pieces of porcelain found in the ruins offer clues to Bahrain's trade connections with China during the 6th to 8th Hijra centuries, corresponding to the 1200 - 1500 A.D. period.