(_Closed._)
On the keystone is a Greek cross within a circle. The outside of the
arch is reached by passing through the Porta S. Sebastiano and turning
to the left. It is formed of two round brick towers and a travertine
stone arch, with grooves for a portcullis; on the outside keystone are
the early Christian emblems of the _labarum_. The Roman Catholic
tradition is that S. John the Evangelist was thrown into a caldron of
boiling oil inside this gate, where the circular church now stands.
Opposite is the Church of S. John, Port Latin.