The Porta Nigra was built in grey sandstone between 186 and 200 CE. The original gate consisted of two four-storied towers, projecting as near semicircles on the outer side. A narrow courtyard separated the two gate openings on either side. For unknown reasons, however, the construction of the gate remained unfinished. For example, the stones at the northern (outer) side of the gate were never abraded, and the protruding stones would have made it impossible to install movable gates. Nonetheless, the gate was used for several centuries until the end of the Roman era in Trier. It serves as an entrance to town.
In Roman times, the Porta Nigra was part of a system of four city gates, one of which stood at each side of the roughly rectangular Roman city. The Porta Nigra guarded the northern entry to the Roman city, while the Porta Alba (White Gate) was built in the east, the Porta Media (Middle Gate) in the south, and the Porta Inclyta (Famous Gate) in the west, next to the Roman bridge across the Moselle. The gates stood at the ends of the two main streets of the Roman Trier, one of which led north-south and the other east-west. Of these gates, only the Porta Nigra still exists today.
Pantheon is derived from the Ancient Greek “Pantheion” (Πάνθειον) meaning “of, relating to, or common to all the gods”
Cassius Dio, a Roman senator who wrote in Greek, speculated that the name comes either from the statues of so many gods placed around this building, or from the resemblance of the dome to the heavens.
Between 354 CE and 361 CE a huge triumphal monument was erected next to the camp and city. Contemporary reports suggest that Emperor Constantius II had it built to commemorate his victories. When the remains of Carnuntum disappeared after the Migration Period the monument remained as an isolated building in a natural landscape and led Medieval people to believe it was the tomb of a pagan giant. Hence, they called it “Heidentor” (pagan gate).
The three vaulted arch composed the western gate of the city, at the beginning of the decumanus maximus and the end of the road coming from Lambaesis.
The arch reaches a height of 12 metres, with a central arch of 6 metres in height which permitted the passage of vehicles that have left deep ruts in the ground under the archway).
The lateral arches, each 3.75 metres high, were reserved for pedestrians. Above the lateral arches on both sides are deep rectangular niches, framed by aediculae with smooth-stemmed Corinthian columns of coloured marble supported by shelves. The niches were designed to hold statues which are now lost. The whole assemblage of each lateral arch and niche was framed by two red corinthian columns, detached from the walls and supported by pedestals. The entablature that runs across the wall above the lateral arches, protrudes above the columns and a curvilinear pediment rests on it in turn. The attic must have been surmounted with a group of monumental statues.
Other sculpture was added to the arch in later times. This includes a statue of Mars and one of Concordia erected under Emperor Septimius Severus by Lucius Licinius Optatianus, on the occasion of his election as flamen-for-life of the colonia.