Porta Nigra by Husker_Jeff
According to the tourist literature, the Porta Nigra is the largest surviving Roman city gate north of the Alps. It's hard to believe this structure is around 1800 years old. The Porta Nigra is striking, for sure. You may purchase a combination ticket for both the Porta Nigra and the City Museum, which I highly suggest since it is only 6 Euros.
Climb the steps of the Porta Nigra and you will have a nice view of Trier.