Kalenderhane Mosque, formerly the Church of the Theotokos Kyriotissa, is a captivating monument where Byzantine, Latin, and Ottoman layers intertwine. Hidden near the Aqueduct of Valens, it once served Crusaders and later housed Sufi whirling dervishes. Inside, visitors can admire rare 12th-century Christian frescoes, including a unique Dormition scene, alongside elegant Ottoman features like the mihrab and minbar. Its architecture blends Middle Byzantine design with surprising Gothic details, making it one of the most atmospheric sacred spaces in Istanbul. Perfect for culturally curious travelers, this mosque-church is a quiet window into the city’s spiritual and artistic evolution over 1,500 years.