Rustaq fort, built four centuries prior to the dawn of Islam in Oman, is an imposing structure built on three levels, containing separate houses, an armoury, a mosque and four towers. The tallest tower stands over 18.5m high and has a diameter of 6m. Hazm Fort is an outstanding example of Omani Islamic architecture and was built in 1711 AD. The fort's roof is built on columns, and contains no wooden supports. Its walls can withstand great impact, being no less than 3m thick.