The thing is called Auenwanderweg and is from the castle to the moor the perfect bypass for Ahrensburg. So you do not have to cross the city, but at the creek and in the countryside. Contact with traffic is marginal - what more could you want.
Very nice bypass of the busy Ostrings Ahrensburg. Along the Hopfenbach you can drive on scenic trails from Ostring at Ahrensburg to Hamburger Straße in Ahrensburg West. By the way, the Hopfenbach Moor is also worth seeing.