This excavation, which can be visited free of charge, is a little off the beaten track of the Roman city, which is subject to a charge. Access is via the private path slightly to the west (above) the museum grounds, but which is explicitly open to cyclists.
Carnuntum was a large city with up to 50,000 inhabitants in ancient times. That is more than the surrounding communities of Hainburg, Bruck Neusiedl and Parndorf have in common. Some of the buildings in the Roman city have been painstakingly rebuilt and a visit to the Roman city right next to it is definitely worth it. I find the fully functional thermal baths particularly impressive with their many different water basins, complex water treatment and floor / wall heating.