Trier, which is in southwest Germany and close to Luxembourg, is the oldest city in Germany and was an important administrative center for the Western Roman Empire. The city is very walkable, and I was able to get everywhere on foot with the help of Google Maps. A full, packed day is enough to cover the major sites and take in the many fascinating ancient Roman ruins. I managed to tour the Porta Nigra, which was a city gate and later a church - Trier is a must-visit for European history buffs and UNESCO World Heritage site lovers!