Before I go to the cool part, about the ancient Roman ruins, bear with me while I tell you a few words about Trier: it's cute. It's the fluffy kitten of the tourist attractions in Germany. It has an attractive combination of history and bustling modern activity and, most unusual about a German town, it seems to have a sense of humor about itself (don't ask what that means, just trust me on this).
I guess you'd have to keep high spirits and a constant good mood, when you're that old: Trier is the oldest city in Germany, founded probably around 3,000 BC, as a small Germanic village. It started its successful career in 16 BC, when the Romans founded Augusta Treverorum on the banks of the river Moselle. The place had been under Roman domination since around 50 BC, when Caesar meddled with the local affairs.
Colonnades on the upper level of Porta Nigra
The first major building project the Romans undertook in the area was a wooden bridge over the Moselle River, which was replaced by a stone one in 150 AD. The ancient stone foundation still bears today's modern bridge in Trier.
The Romans considered the strategic importance of the city location, and Emperor Augustus made Augusta Treverorum into the capital of the province Belgica. Fortified walls were added, as the situation in the area got more and more confusing in time.
The only part that survives today from those ancient walls is the famous Porta Nigra. Built around 180 BC, it was one of the four access gates to the city, embedded in massive walls. The other tree gates, destroyed during medieval times, are known as Porta Alba (the white gate), Porta Media (the middle gate) and Porta Inclyta (the famous gate). It is however not clear whether Porta Nigra had its current name (black gate) during ancient times, as it seems more likely it was called like that after a series of fires blackened its surface.
The gate's imposing aspect was designed mostly to shock and awe traders, to make them spread the word that the city's defenses were impenetrable. Fact is, the fortifications were built during relatively peaceful years, and we know for sure that Porta Nigra remained unfinished (possibly other parts of the walls as well), since the mechanism for movable gates was never installed. Maybe that's the reason why Trier was a rather easy prey for the Germanic tribes, who invaded and sacked the city on several occasions, and for Roman armies, who moved to and fro several times during the conflicts from the 3rd and 4th century.
Porta Nigra is most famous for its construction style, which did not use mortar, the stuff that made other Roman constructions the best in the world: instead, the sandstone blocks are held together by iron clamps, fastened into the stones with molten lead. The unusual building method would ultimately lead to the downfall of the fortifications, as, during the Middle Ages, the locals would tear them down in order to re-use the precious metal. Stones, of course, were re-used as well.
The metal clamps that made Porta Nigra such a famous building
Porta Nigra avoided becoming a victim of the scavengers thanks to a hermit monk, by the name of Simeon, who made it his home. As Simeon was sanctified after his death, Porta Nigra was transformed into a church and the Simeonstift monastery was built close by. The former windows of the Roman structure were enlarged and turned into access gates to the upper levels, which now contained rooms for monks.
Christian addition to the inside of Porta Nigra
In 1802, Napoleon Bonaparte's army invaded Trier and destroyed most churches and monasteries; however, instead of destroying the church located inside Porta Nigra, they decided to tear down the later additions and restore the building to its original Roman form. That's pretty much what today's visitors can see.
Trier continued to grow and became more important during the third century, when the Roman empire faced one of its worst crises. Various minor usurpers and ephemeral break-away leaders made Trier their residence, until Diocletian's reorganization of the Empire. Trier became the residence of the Western caesars, first for Maximian and then for Constantius Chlorus, but it was Constantine the Great who undertook the biggest construction projects and brought fame and glory to the city.
The most famous of Constantine's buildings in Trier is Aula Palatina, also known as the Basilica of Constantine. Built around 310 AD, part of the palace complex at the time, it has the largest extant hall from antiquity. It's just one huge room, 67 m long, 26 m wide and 33 m tall, but the Romans still had style, even when constructing such massive single-room structures.
First of all, the gigantic room was endowed with a hypocaust – which is a heating system for the walls and floor. I don't even want to think how much fuel you had to burn to heat up that room. Then, they used an optical illusion: all windows are progressively smaller, to create the impression of depth (as if the building wasn't big enough already).
During Roman times, when it was used as the throne room, the building was surely richly decorated with statues, paintings and mosaics, but nothing survives to this day. The basilica was turned into the residence of the bishop in Trier during Medieval times, and it is now a stern and austere Protestant church – style that actually seems to suit the building, as it underlines it's huge dimensions.
The Imperial Baths
Constantine the Great continued to pay attention to Trier, as it was the chosen residence of his mother, Helena. More buildings were added, such as a new stadium, built close to the existing amphitheater, to host chariot races.
Still standing today from the same period are the ruins of the Imperial Baths, the largest ancient Roman bath structures to be found outside of Rome. The construction project was probably started by Constantius and finished during the reign of Constantine.
Next to the actual baths (thermae) there's the mandatory field (palaestra) for athletic exercises and the occasional gossip that had to be taken in the open, far from obnoxious eavesdroppers. The total surface covered is of 260 by 145 m. The surviving masonry work proves that special attention was paid to details and aesthetics, with bricks and limestone blocks arranged in various patterns.