Our primary interest is St. Paul's Catacombs and St. Paul's Grotto. It's a pleasant 15 minute walk from the bus stop. When we arrive we're pleased to learn that the €5 admission includes the catacombs, the grotto, WWII Shelters and the Wignacourt Museum.
The catacombs are dated from the 4th to the 9th centuries, AD. There is much better lighting than I remember in the Roman catacombs we visited 10+ years ago. William and I meander through the catacombs for 45 minutes stopping to see the gravesites. Some of the passages are tight and/or low. William's hat saves him from scraping his head at several points along our journey.