…that the St. Pauls arched tunnel is one of the four remaining city gates still in existence?

It was once part of the old Roman defence wall (a defence wall that arches around the Seminary garden on Cathedral Hill).
Though its look has changed many times over the centuries, including the Baroque façade that it still has today it is still the oldest of the four remaining gates! Called the “Dark Vaulted Tunnel” (the only light coming from the entrance and exit) until the 19^th century when arches were  built into the river side of the tunnel giving it more light. For a long time it was the only way to enter the Bishops city by land. The Sacristy of St. Pauls parish church stands over the tunnel thus saving it from demolition.