3 Must-See Hop-off AttractionsIf your time in Trier is limited then be sure to visit these must-see attractions. First on the list at Bus Stop 1 is the Porta Nigra, the sandstone coloured Roman city gate that began construction in 186 AD. At Bus Stop 3 you'll be able to take in the beautiful surroundings of the Barbara Baths, dating all the way back to the 2nd century. If you want to embark on a history lesson and learn about Trier's heritage then hop off at Bus Stop 6. Here you'll find the house in which the famous German philosopher and sociologist, Karl Marx, was born in 1818.