PORTA NIGRA
The Porta Nigra is a former Roman city gate on Porta Nigra Square that was built from 170 AD and is a landmark of the city of Trier.
Construction of the city gate began in 170 AD as the northern entrance to the city of Augusta Treverorum (Augustan city in the land of the Treveri). The dating of the gate was long disputed and ranged from the 2nd to the 4th century A.D. In January 2018, a dendrochronological examination of wooden remains from the city wall was able to pinpoint the start of construction to the year 170 A.D., as these had been felled in 169/170.
There are signs carved into the stones in various places, some of which are upside down. These are probably stonemason's marks that help to reconstruct the construction of the gate. The marks in the west tower contain dates, albeit without the year, so that it is not possible to date the Porta Nigra absolutely in this way. However, the marks can be used to estimate the time it took to build the gate, as they labelled several continuous and superimposed ashlars. If these times are extrapolated to the entire structure, taking into account a sensible subdivision into construction lots and excluding the winter as a construction period, the Porta Nigra would have been completed as a shell within two to four years.
The building, which was begun under Emperor Marcus Aurelius, was never finally completed. For example, the holes for the door hinges of the gates had already been prefabricated. However, the bosses of the unfinished ashlars still protrude into the axis of rotation of the gates, meaning that a movable gate could never be installed. (Source: Wikipedia)