Arriving in Trier in the dark was never part of the plan.

Ten days in Germany, in three Rhineland cities, train tickets, hotel reservations: I was organized, with a detailed, day-by-day itinerary. But after unexpectedly spending more than two hours in the Düsseldorf train station searching for misplaced luggage, I’d missed the early trains to Trier.

As I hurried down the town’s main street at 10 p.m., coaxing my suitcase over cobblestones, the sight of a glowing pink hulk looming in the gloom stopped me in my tracks. The ghost of the Roman Empire, hovering in the night?

Yes, sort of. It was the Porta Nigra, Trier’s best preserved Roman ruin, illuminated at night by spotlights. The second-century gate — made with massive sandstone blocks, 98 feet high and twice as wide — is a UNESCO World Heritage Site.

In daylight, the Porta Nigra is a dirty gray, mottled by 1,800 years of weather, which is why it’s called the “black gate.” But with that first shimmering apparition — the gate at night as it might have looked lit by Roman torches — I understood the appeal of Trier, an ancient city on the winding Moselle River filled with imposing wonders. The Porta Nigra is one of Trier’s nine World Heritage Sites, historic monuments all.

I returned early the next morning, and groups of tourists were already milling around in front of the Tourism Office, sipping coffee, perusing brochures and waiting for their guides. I went to the office to pick up a city map and ask about guided tours.

The most popular tours, the desk clerk said, were the afternoon bus or riverboat excursions to the wineries and vineyards along the Moselle River. In the meantime, I might want to try the Roman Ruins tour led by a centurion in a breastplate and helmet; or the Toga Tour of Roman Trier, with guide and visitors clad in what looked like ribbon-trimmed sheets. The gladiator-led adventure to the Amphitheater, complete with imagined battle scenes, seemed designed for kids with a taste for gore. The “Devil in Trier” promised a spooky immersion in the Middle Ages’ darkest years. “Beware of witchcraft!” she advised, smiling.

The tour I picked, walking through the old town streets, sounded dry by comparison, but focused on history. Meanwhile, Chinese tourists made a beeline for a modest three-story pink house with a historic marker on the wall, and a “Euroshop” (a 99-cent store) on the ground floor.

“They want to see where Karl Marx grew up,” said Elke Schmeier, a guide. “His house and the museum down the street are the first thing Chinese tourists ask about,” she said. “They’re surprised when they hear that Marx came from a middle-class family.”

The Chinese weren’t the only visitors learning something new. By now, I realized that Trier, the lively, modern German town, and Treveris, the Roman city founded by Augustus Caesar in 17 B.C., were one and the same. The difference was geography.

Trier today sits comfortably in the midst of Western Europe, minutes from Luxembourg and close to France, all European Union members. But Treveris, the capital of the Roman Empire’s northwest sector, was an outlier on the frontier, a bulwark on the border between civilization and hostile barbarian hordes.

Still, the Romans built for the ages, even in a trackless forest. Trier was surrounded by a defensive wall and its streets were surveyed and paved. Residents lived in modest villas and enjoyed monumental public buildings, with a temple (later a church), imperial throne room and a 20,000-seat amphitheater for gladiator games. Two enormous public baths, supplied by 6 miles of bricked, underground tunnels, boasted heating, cold and hot clean water and drains for steam rooms, baths and massage rooms.

“Look at this Roman cement,” said Schmeier, tapping the spaces between bricks in one of the tunnels. “Still good since the fourth century. Now look at the cement in the restored tunnel over there. Eroding after 60 years. What does that tell you?”

Decorative flourishes

As long as Trier was one of Rome’s capitals, it prospered. But the town declined after 316 A.D., when Emperor Constantine II moved the seat of power from Rome to Constantinople (now Istanbul). After the empire disintegrated, in 476 or thereabouts, the medieval church ruled the city, gradually ceding control to regional princes, ambitious power brokers and eventually to nation states.

But building continued. Churches, town halls, houses, prisons and palaces squeezed in beside — and sometimes even on top of — older buildings. Roman pediments and columns gave way to Romanesque churches, then to early and late Gothic cathedrals, followed by Renaissance, Baroque, rococo, romantic, neoclassic, art deco and modern structures.

Old favorites were saved and enlarged. War damage was repaired. Roman blocks were carted away and reused, or saved for restoration. When the bricks wobbled or the paint peeled, the buildings were repaired. And the colors: candy-land pink, baby blue, green, white with gold flourishes, white, timber-framed houses with beams edged in red.

The decorative flourishes on facades, doors and windows — swirls, angles, arches, spirals, scrolls, petals and leaves — were beyond counting.

Especially intriguing was the fourth-century Constantine Basilica. So broad and tall that the Porta Nigra could fit inside, the basilica proved its worth as a Roman hall, church, concert venue and lately, now that it’s equipped with chairs, as a place or tourists to sit and be amazed.

Cathedral atop a church

For real amazement, the Cathedral and Liebfrauenkirche (church) complex took the gold. Significant enough to warrant its own fact-packed tour, the architecture deserved a close look, with enough time to see how the columns, windows and side aisles work together.

The first building on the site was a square Roman structure, possibly a palace.

Converted to a “house church,” it was lengthened at least twice before the Romanesque cathedral was built right on top it, in the 11th century. In the 13th century the early Gothic Liebfrauenkirche was erected next door, adjoining the cathedral. Today the two stand as one, sharing a common wall and cloister.

The tour completed, some of us headed for a favorite local spot, the Weinstube Kesselstatt. A clubby hideaway with dark wood paneling and tables, it was a perfect place to order bratwurst with potatoes and sauerkraut, and a typical wine, a Saar River riesling.

Afterward, I wandered over to the 10th-century Market Square to take it all in again, the Gothic, Classic and Baroque buildings standing cheek by jowl, and in the middle, the stubby stone cross (installed in 958 A.D.) that identified the spot as a bishop-licensed market. You sold your cabbage and onions here or not at all.

Five hundred years later, in the year 1495, the St. Peter Fountain, the base adorned by four blue and white, gold-fringed figures, representing the four virtues, was installed, adding panache to the square.

It took a while coming, but here in Trier, it could last forever.