There's a lot to see here, so I'll name some of the highlights. The large Dome of course, and the baths. And, somewhat out of the center of town, the huge Amphitheatre.
Really interesting also is the regional museum. One room there exhibits large mosaics in good condition that have been found in Trier. At the heart of the museum an ornamented light blue grave monument is the center piece of attraction. How beautiful the streets of Trier must have looked like in Roman Times!
Jorge Sanchez (Spain):
Trier is a stunning town with several Roman vestiges not expected to be found in Germany.
As soon as I arrived to Trier, by train from Saarbrucken, walked perhaps 200 meters and on my left side I found the stunning Porta Nigra, which caused me much admiration.
In that square was located the Tourist Office and the Stadmuseum Simeonstift.
According to a plaque in his tomb besides that museum, Simeon was born in Sicily and went to a pilgrimage to Jerusalem, then became a monk and later a hermit in Sainte Catherine monastery, Sinai. Finally he travelled to Trier, via Rome, and lived as a recluse inside the tower of the Porta Nigra from 1030 until his death in the year 1035.
In the place where he was buried there were constructed two churches in his honor, but Napoleon destroyed them in the XIX century.
After the Porta Nigra I walked to the Cathedral, which is the oldest in Germany. It was open and could visit it. In that cathedral is preserved the Holy Tunic of Jesus Christ, apart from other relics.
Many more amazing buildings I saw during that day in Trier, churches and monasteries. Trier is very rich in history.
I went to the River Moselle, where I saw a Roman bridge. I crossed it and read a sign saying that the Römerbrücke is a UNESCO Welterbe seit 1986.
I also saw a sign in a street informing that Trier is a stop for pilgrims in the Camino de Santiago, Spain (Jakobsweg), via Le Puy-en Velay.
So excited was I exploring the town that suddenly became dark. Then I walked to the Youth Hostel, at about 20 minutes distance from the Porta Nigra, by the River Moselle, to spend the night.
The next day I continued my visits in Trier until I felt that I had exhausted all the highlights, not missing a single one. Finally, at midday I caught a train to Namur, in Belgium, to admire its beautiful Belfry.
Date posted: March 2014 Clyde (Malta):
Trier is the oldest city in Germany and packed with Roman monuments and sites to see. The city's landmark is the Porta Nigra (Black Gate). Close to the Hauptmarkt there are the Dom (which houses the Holy Robe - on display April 13 to May 13) and the Church of Our Lady. Further ahead are the Imperial Roman Baths and the Evangelical Lutheran Church which are worth viewing. A great day trip and worth the overnight stay to see the sites without the tourist groups early in the morning.
Date posted: April 2013 Klaus Freisinger (Austria):
Trier is the oldest city in Germany and there was a time when it was the second most important city in the Roman Empire after Rome itself - and it shows. I have rarely seen a city with such an impressive array of monuments from Antiquity onwards through many time periods up to the present day - a veritable cross section of European history that is remarkably well preserved (sometimes it helps to be off the beaten track). Of course, Roman and medieval monuments dominate the scene, especially the world-famous Porta Nigra (don't neglect to go inside), the cathedral and the Church of Our Lady next door. The city's history is especially connected to Emperor Constantine the Great and the conversion of the Empire to Christianity. I visited Trier for the huge exposition on Constantine on the 1700th anniversary of his marriage in Trier, which was great, but the city itself impressed me even more.
Kevin (America): Trier is full of monuments worth seeing. Often overlooked is the town square just up the street from the Porta Nigra. This square turns into a wonderful Christmas Market during December with some of the best food in the country. The town square is also a street or two from St. Peter's Cathedral which is a great example of Romansque architecture. It houses the holy tunic of Christ. I enjoyed the city immensely. If you want to have a walk that you will enjoy, walk through the Kaiserthermen (Roman Baths) or the Ampitheater. Remember the Ampitheater goes down under as well.   Ian Cade (England):
I liked Trier and was surprised by the amount of high quality monuments in the city. The Porta Negra is very impressive, the Dom is stunning for its importance and its holy relic, a tunic supposedly worn by Christ. Then there is the Basilica of Constantine which is impressive for its age, the massive interior and architectural importance as being a forbearer to the Romanesque movement. The Amphitheatre is a little way out side the city walls but is quite large and provides a great place to sit and have a sandwich or run around like a school child (I opted for both with the emphasis a bit too much on the later!)
Not part of the UNESCO site is the birth place of Karl Marx! There is not much there, but it is nice to see none the less.
Trier has enough to occupy you for a day trip and makes a great one from Luxembourg as it is only about 30 minutes by train.
Jennifer (America): Trier is one of the most beautiful cities I've ever seen.   bob zijlstra (netherlands): some 50 kilometers southwest of trier there is the HUNNENRING, a celtic fortress from 300 BC.
Julius Caesar came to the Mosel area and won the battle with the Treveri. The fortress had become useless
But it is very interesting, 2 km wall, 10 meter high.
It should become a Unesco World Site
Jim Humberd (usa): Germany’s capital of Roman Antiquity is a magnificent surprise. Located astride the Mosel River about six miles from the Luxembourg border, Trier is little known to most travelers who vacation in Europe, but it should be one of Germany’s most visited cities.
The oldest city in Germany as well as one of the oldest and most interesting on the continent, one motto says, “Before Rome, there was Trier.” Legend says that Trier was founded in 2000 BC (1300 years before Rome), and the Romans didn’t arrive until 14 BC. In 41 AD the Roman writer, Pomponius Mela, called Trier “urbs opulentissima,” a very opulent city.
… … in Trier while we walked just 2,000 steps, we witnessed and enjoyed 2,000 years of history.
From my book Invitation To Germany (google)
Isabelle Rotsch (Germany): The best place to have a look at the beautiful skyline of Trier is at night on the "Petrisberg". All the sites are illuminated and it is just great. Best time is August; warm and clear nights. I have to know, it's my hometown!
Have you been to Roman Monuments, Cathedral of St. Peter and Church of Our Lady in Trier? Share your experiences!