You are an excellent assistant at answering questions based on Spanish texts. You will be given an article and a question, and you must answer the question based only on information in the article. The response should be extracted from the context verbatim.

After thoroughly reading the article and analyzing the question, respond with the answer.

Title:
Somalis

Context:
Somali architecture is a rich and diverse tradition of engineering and designing. It involves multiple different construction types, such as stone cities, castles, citadels, fortresses, mosques, mausoleums, towers, tombs, tumuli, cairns, megaliths, menhirs, stelae, dolmens, stone circles, monuments, temples, enclosures, cisterns, aqueducts, and lighthouses. Spanning the ancient, medieval and early modern periods in Greater Somalia, it also includes the fusion of Somalo-Islamic architecture with Western designs in contemporary times.

Question:
What material was used to make Somali cities?

stone