Short answers, pulled from the story.
Trier was founded by the Romans around 16 BC as Augusta Treverorum, making it older than any other surviving German city. It served as a residence of the Western Roman Emperor from 293 to 395 AD and was one of the four capitals of the Roman Empire during the Tetrarchy period.
Several Roman and medieval monuments in Trier are listed as a UNESCO World Heritage Site because of the city's significance during the Roman and Holy Roman empires. These include the Porta Nigra, the Aula Palatina, Roman baths, the 2nd-century Roman bridge over the Moselle, and Trier Cathedral.
Karl Marx was born in Trier in 1818, during the period when the city was part of the Prussian Rhineland. The Karl Marx House in Trier now operates as a museum displaying his letters, poetry, photographs, and rare first editions of his works.
The Trier witch trials took place between 1581 and 1593 and are considered one of the four largest witch trials in German history, and possibly the largest in European history. Within the city limits alone, around 368 people were executed; a total of 1,000 across the whole diocese has been suggested but not confirmed.
At its height in the 4th century AD, Trier had a population of around 75,000 people, and possibly as many as 100,000, placing it among the largest cities in the Roman Empire at that time.
The Porta Nigra, meaning Black Gate, is a Roman city gate dating from the period when Trier served as an imperial capital. It is the best-preserved Roman city gate north of the Alps and is part of Trier's UNESCO World Heritage listing.