Molla Zeyrek Mosque is one of Istanbul's great Byzantine survivors. It began as the church of the Monastery of Christ Pantokrator in the 12th century, grew into a complex with a public church and imperial burial chapel, and later became a mosque under the Ottomans.
The building has been altered less radically than many former churches in the city, so it remains one of the best places to read middle-Byzantine architecture in stone and brick. Visit by day or around dusk, and be respectful of prayer times.