The concept of protecting the Citadel of Islam (Hifẓ Bayzat al-Islam), is common in the lexicon of Shiʿi political jurisprudence. It refers to those Islamic principles and foundations without which the existence of Islam and the Muslim community would be in danger. During the Occultation Era, Shiʿi mujtahids, as general deputies of the Hidden Imam, are believed to commit their utmost effort to fulfilling this sacrosanct responsibility. The meaning and applications of the concept have been evolved throughout the Shiʿi history. A noteworthy case of protecting the citadel of Islam is the position of Sayyid Abu’l-Ḥasan al-Iṣfahānī (1867-1946) and Mirza Muḥammad-Ḥusayn Nāʾīnī (1862-1937), vis-à-vis the Najaf seminary and the newly established Hashemite dynasty in early twentieth century. This paper aims to shed more light on the concept of the Citadel of Islam, its implications in Shiʿi political thought, and contemporary applications.