The Arke Bed & Breakfast can easily be reached  using the public transport system.   Buy tickets or travel cards before taking buses/the underground/local trains, as you cannot pay on board.   There are several types of travel cards available and basic tickets cost 1 Euro each.  Tickets are valid for any number of trips by bus or local train within 75 minutes of stamping, but on the underground a ticket must be stamped for each one-way journey.  Children up to age 10 travel free.   Tickets can also be purchased at the Arke Bed & Breakfast.

From Fiumicino 'Leonardi da Vinci' airport:

Take the first train to Trastevere Station.  From there, take the first train going to any of these stations: Civitavecchia  - Viterbo -  Bracciano - Cesano.  Get off at the first stop (S. Pietro Station)

The B&B is 100 metres from the station.

From Ciampino airport:

Take the bus either to Ciampino train station or to 'Anagnina' underground station (this bus runs approximately every 40 minutes Mon-Fri, every hour weekends/holidays).    From there, take the first train to Termini (Rome's main station) and then take Bus 64.     Get off at Via del Crocifisso.   The B&B is 10 metres from the bus-stop.

From Termini train station:

Take Bus 64.   Get off at Via del Crocifisso.   The B&B is 10 metres from the bus-stop.

By car:

From the GRA (Grande Raccordo Anulare)  - Rome's ring road -  take the Via Aurelia (n° 1) exit for the city center, then follow the directions to St. Peter's.