We passed this often during our stay in the Marais. I thought it was strange to see this old building material just "abandoned" near a much newer structure. Upon investigation we found
Enceinte de Philippe Auguste Restes de la tour Montgomery de la polerne Saint-Paul, constuite du XII siecle. This is all that was left of the ancient structure and I am happy that it was preserved. Paris is full of many marvels from the past. I wish I knew of them all.