We were there on a cold, wet Saturday in winter and there was nothing really going on in Xanten, so parking was super easy. We parking in a free lot within the city wall near the Rathaus. There are quite a few parking areas in Xanten, some free for a short time and others are pay parking. Outside the wall is a rather large parking lot near the Nibelungen Platz and roundabout. It is just a short walk from any of the parking lots in Xanten.
Additionally, if you are going to the Archaeological Park, you can either park your car there and walk to Xanten or park at Xanten and walk to the park. They are both rather close to each other.
A small ferry operates from Xanten across the river, it only takes foot passengers and of course their bikes. Across the river at that point, there is a small sandy shore line were you could sunbathe weather permitting.
The Niederrhein airport is just 10-15 minutes away by car from this place, it is very small airport near by, only few small airlines use it, but you can get very cheap flights to several destinations around Europe
I guess there are some buses that going here, but the only easy way to get here from the airport is by car.