The Arch of Janus was on the communication ways of the Velabro.
On the map, you can notice the double blue line going under the Arch of Janus.
It is the identification of the great sewer (cloaca maxima) which
stretched between the Capitol, the Palatine and the slopes of the Esquiline.
The Cloaca Maxima allowed to drain the swampy valley.
Still today you can see the outlet of the sewer on the side of the Tiber.