It is extremely easy to get into the city, with the S-bahn taking only 11 min. and one coming by every 15 minutes from track 1, at the Regional train station, under Arrivals B-1, in terminal 1. Buy a group day ticket. Ride 3 stops and you are at the main train station, HBF (beautiful 124 year old massive, historic station). Or instead, ride 5 stops and you are at the Hauptwache stop, in the center of downtown and a 5 min. walk from the Römer.