Kilwa is a small island off the east coast of Africa, part of what is now Tanzania. To distinguish it from the other sites named Kilwa, this island is referred to as Kilwa Kisiwani, which means Kilwa on the island. Its position in the Indian Ocean is just less than three miles off the shore of Tanzania, so it can be seen from the mainland. The island is separated from the mainland by 3 kilometers wide channels.

It was once the most famous trading post in East Africa. The Swahili wealth owner of the Island sold it to Ali bin Al Hasan, the founder of the Shiraz Dynasty in the 9th century. From 11th to early 15^th Century, Ali bin Al-Hasan managed to create a powerful city, Kilwa Kisiwani. It was a major trading center along east African coast, and inland as far as Zimbabwe. Trade was mainly in gold and iron from Zimbabwe, ivory and slaves from Tanzania, and textiles, jewellery, porcelain, and spices from Asia.

In 1331, a Moroccan Abu Abdullahi Ibn Batuta visited Kilwa and the outside world came to know about it.  Kilwa declined when the Portuguese arrived. At first, the Portuguese, under the leadership of Vasco da Gama, threatened to attack the city unless a tribute was paid. Eventually, they decided that they wanted Kilwa's prosperity for themselves and took the island by force in 1505. They constructed a large costal fortress called Gereza. The island was under Portuguese control until an Arab mercenary and his men liberated the island in 1512.

Kilwa never recovered its former prosperity. The Portuguese still occupied much of the East African coastline. There was minimal trade; slaves and ivory were circulating through Kilwa's port. The new Sultan made merchants sign an oath to only trade with Arabs and not Europeans.

For 300 years, Kilwa was a forgotten city-state until when Europeans began to fight over it as a military base. In the eighteenth century the French occupied it and then from 1886 until World War 1 the Germans. In 1840, Kilwa was abandoned.  

Serious archeological investigation began in the 1950's. In 1981, Kilwa Kisiwani was named a UNESCO World Heritage site. Some sites in the area are: the Great Mosque, which was considered to be the largest in East African Coast; The Great House, which is believed to be the house of Imam or Sultan; The Small Domed Mosque, the best preserved and most ornamental in Kilwa; Mkutini Palace, built with great walls triangular in shape; and Gereza which was built by Portuguese, and the remarkable ruins.

Reference:

http://www.mnsu.edu/emuseum/cultural/oldworld/africa/kilwa_kisiwani.htm
www.utalii.com/Off_the_normal_path/kilwa.htm
http://en.wikipedia.org/wiki/Kilwa_Kisiwani