stone monuments scattered around the country today. Houses were built of dressed stone similar to the ones in Ancient Egypt, and there are examples of courtyards, and large stone walls, such as the Wargaade Wall, enclosing settlements. The peaceful introduction of Islam in the early medieval era of Somalia's history brought Islamic architectural influences from Arabia and Persia, which stimulated a shift in construction from dry stone, and other related materials, to coral stone, sun-dried bricks, and the widespread use of limestone in Somali architecture. Many of the new architectural designs, such as mosques, were built on the ruins of older structures, a practice that would continue over and over again throughout the following centuries. Dhulbahante garesa In the official Dervish-written letter's description of the 1920 air, sea and land campaign and the fall of Taleh in February 1920, in an April 1920 letter transcribed from the original Arabic script into Italian by the incumbent Governatori della Somalia, the British are described taking twenty-seven garesas or 27 houses from the Dhulbahante clan: Tanzania Engaruka is a ruined settlement on the slopes of Mount Ngorongoro in northern Tanzania. Seven stone-terraced villages comprised the settlement. A complex structure of stone channels along the mountain's base was used to dike, dam, and level surrounding river waters for irrigation of individual plots of land. Some of these irrigation channels were several kilometers long. The channels irrigated a total area of . Swahili States Farther south, increased trade with Arab merchants, and the development of ports, saw the birth of Swahili architecture. An outgrowth of indigenous Bantu settlements, one of the earliest examples is the Palace of Husuni Kubwa, lying west of Kilwa, built about 1245. As with many other early Swahili buildings, coral rag was the main construction material, and even the roof was constructed by attaching coral to timbers. The palace at Kilwa Kisiwani was a two-story tower, in a walled enclosure. Other notable structures from the period include the pillar tombs of Malindi and Mnarani in Kenya and elsewhere, originally made of coral rag, and later from stone. Later examples include Zanzibar's Stone Town, with its famous carved doors and the Great Mosque of Kilwa. A visitor in 1331 AD considered the Tanzanian city Kilwa to be of world class. He wrote that it was the “principal city on the coast the greater part of whose inhabitants are Zanj of very black