One of the most atmospheric churches in Cairo, this round Greek Orthodox church is actually a reconstruction of a 7th-century church that almost burned to the ground in the 19th century. It is built on top of the north tower of the old Roman fortress of Babylon and within the grounds of the Monastery of St. George. If you can, it is well worth descending underneath the church to see the Roman foundations there. For decades, these were full of water, until a huge USAID-funded project started in the 1990s allowed the drainage of built-up ground water and made it possible to see the original Roman stonework. There is a nice relief of St. George on the second landing as you climb the stairs to the church, and underneath (though still above the Roman foundations) there are small chapels and displays that are worth visiting if they're open.