Trier or Treves - Germany's oldest city. Can you believe it already existed when Jesus was living? Numerous famous people were born here (like Karl Marx) or left their footprints (like Emperor Constantine the Great). Lots of remanents from ancient Roman days like this one: The "Porta Nigra" one of the ancient city gates was built in the year 180 and it's now an UNESCO world heritage site. The stone slabs were cut by bronze saws powered by mill wheels (some cutting traces are still visible) and put together without mortar. Instead they used iron clamps to hold the structure together.

Trier - Porta Nigra

This is the Porta Nigra in Trier. The light on the inside (green in this pic), changes constantly giving a different pic each time. en.wikipedia.org/wiki/Porta_Nigra