Complex sentence: The travertine arch that constituted the eastern entrance was on what is now the Piazza del Collegio Romano-this has also been demolished, but its western pillar is built into the building on the corner of Via del Gesù.
Split into simpler sentences: The travertine arch that constituted the eastern entrance was on what is now the Piazza del Collegio Romano. It has also been demolished, but its western pillar is built into the building on the corner of Via del Gesù.