Cairo holds one of the greatest concentrations of historical monuments of Islamic architecture in the world. The areas around the old walled city and around the Citadel are characterized by hundreds of mosques, tombs, madrasas, mansions, caravanserais, and fortifications dating from the Islamic era and are often referred to as "Islamic Cairo", especially in English travel literature. It is also the location of several important religious shrines such as the al-Hussein Mosque (whose shrine is believed to hold the head of Husayn ibn Ali), the Mausoleum of Imam al-Shafi'i (founder of the Shafi'i madhhab, one of the primary schools of thought in Sunni Islamic jurisprudence), the Tomb of Sayyida Ruqayya, the Mosque of Sayyida Nafisa, and others.