Back in the city from Palestrina I left the metro before reaching the centre to visit San Giovanni in Laterano, Rome's cathedral and the seat of the Pope as bishop of the city. The building was reworked by Borromini in the mid-17th century, but much of the inside dates from 1600. I'd describe it as impressive and stern rather than beautiful, but certainly interesting and with rich decoration around the altar. The entrance doors are absolutely massive. The inside was rather spoiled by rows of cheap and elderly plastic chairs, which were being moved around noisily by a workman.
Paid a couple euros to go into the very pleasant and peaceful cloisters, decorated in the 13th century, and displaying remains of earlier stonework from the site.