Well! The British Museum seems to have quite a few engravings by Giovanni Battista De’Cavalieri online. Browsing them here, I quickly see that some come from a 1569 book entitled, promisingly, Urbis Romae aedificiorum illustrium quae supersunt reliquiae, i.e. Remains of famous buildings of the city of Rome. It contains some fascinating images.
Here’s the one of the Septizonium: