Most guidebooks to Germany give Trier only a page or two, if that much. No standard package tour to the country includes it. And even intrepid do-it-yourself travelers who map out their own itineraries might tend to pass it by.

Considering its size and location, perhaps that is understandable, though a shame. Snuggled safely and almost invisibly between rolling vineyard-covered hills at the confluence of the Moselle, Saar and Ruwer rivers, within easy walking distance of the Luxembourg border, it does seem far off the beaten path. And with a mere 100,000 population, Trier is on the bottom of what Germans call their list of "cities."

But what a city!

For Trier--which this year is celebrating the 2,000th anniversary of its official founding as "Augusta Treverorum" in 16 B.C.--is the "Rome of the north," and in its heyday was as important and splendid as Rome itself.

In his "De Situ Orbis," a description of the then known world, 1st-century Roman geographer Pomponius Mela called it the "urbs opulentissima"--the most opulent city--of the empire. It was from Trier that a succession of six caesars governed their far-flung western European realm and ruled over Britain, Gaul and Spain. It was where Constantine became "the Great" and where he resided until A.D. 330, when he moved eastward to build a new capital at Byzantium, later known as Constantinople.

A history and reputation like that alone make Trier worth a detour, especially in this year of exhibitions, festivals, concerts and pageants. What makes it worth a special trip any time, however, is that the stony legacy of the Roman Empire has survived the centuries remarkably unscathed by more wars than people care to remember and that all has been impeccably preserved or restored.

The city is a palette of remnants: the Romerbrucke, a Roman bridge across the Moselle, built almost two millennia ago, whose ancient pillars and arches now support modern traffic; the Imperial Bath, or Kaiserthermen, completed in the 4th century; a 1st-century amphitheater seating 25,000; the Aula Palatina, the palace and audience hall of Emperors Constantine I, Valentian I and Gratian, which is almost like new and dwarfs virtually every structure in Rome. Above all, there is the Porta Nigra--the Black Gate--to the city and, in a sense, to the empire itself. Some 100 feet high and 120 feet wide, it is an impressive symbol of Roman might and power, and one of the finest, best-kept examples of ancient public architecture anywhere.

Although Trier is throwing its 2,000th birthday bash as Germany's oldest city this year, its origins go further back than that. No one knows how old it really is. To be sure, Augustus chartered it in 16 B.C., but it had been conquered 42 years earlier by his grand-uncle, Julius Caesar, and the records make it clear there had been something worth conquering: a Celtic town dating from around 400 B.C.

It is from that tribe of Celts--the Treveri--that the Romans took the city's name, Treverorum, and why the French call it Treves. But there are legends, as well as excavations, to suggest that the name derives from a settlement established by an Assyrian prince named Trebeta 2,000 years before Julius Caesar came along. Trebeta allegedly was the stepson of Babylonia's mythical Queen Semiramis, noted for her beauty, wisdom, conquest of many lands and for vanishing from earth in the shape of a dove after a long and prosperous reign.

"I wouldn't place too much credence in that," says a town-booster and official of the municipal tourist office. "It's probably a legend concocted by one of my medieval predecessors." On the Rotes Haus, one of numerous lovely buildings surrounding the Hauptmarkt, the main market square, there is a chiseled inscription in Latin, itself four centuries old: "Trier existed 1,300 years before Rome. May it enjoy eternal peace."

At any rate, by A.D. 40 Pomponious Mela was gushing about the city, and toward the end of the 3rd century, when Emperor Diocletian made it the capital of Gaul and the western part of the empire, it had a population of 90,000--almost as large as today.

In the 3rd and 4th centuries Trier vied with Alexandria and Rome as one of the richest, most affluent centers in the empire--a fact reflected in the splendor and grandeur of its buildings and facilities:

* Constantine's Imperial Baths--with their network of hot and cold water basins, drying rooms, dining halls and forum--cover an area of more than 400,000 square feet--large enough to accommodate four football fields.

* The Porta Nigra is by far the largest, most impressive and architecturally stunning city gate ever built in Europe.

* The Aula Palatina, in which the emperors established their throne room, is second only to the Pantheon as the largest Roman public building to have survived the centuries--240 feet long, 94 wide and 108 from floor to its wood ceiling.

With so much of its glorious past preserved and so much more being found--one can buy Roman coins, artifacts and stones in virtually every antique shop--Trier would take weeks to explore. There are two shortcuts.

Spend a half day or so in the Landes Museum, which abounds with antiquities, then accompany one of the trained, English-speaking guides (the most knowledgable I have found anywhere in Germany). Tours can be arranged at the Verkehrsamt, the tourist office in the 11th-century Convent of St. Simeon, adjacent to the Porta Nigra. Two-hour tours for families or groups of up to 35 cost around $20, every additional hour another $6. For those who prefer to do it on their own, the tourist office also has a number of excellent English guidebooks, all translated by linguists with a fluent command of the idiom, which makes reading them a pleasure instead of the usual riddle-solving chore.

There is more to Trier, however, than its Roman legacy. It is one of the wellsprings of Christianity, indeed the site of the Roman Empire's acceptance of Christianity as the state religion, because of the conversion of both Constantine and his mother, St. Helena.

Not only was St. Ambrose born there in 340, but it is the repository of one of Christendom's most sacred relics--the Holy Shroud, believed to have been worn by Jesus on his way to the cross. It was brought to Trier by Helena from her pilgrimage to Jerusalem in the early 4th century and is enshrined in the treasury of the Petersdom, St. Peter's Cathedral, where it is placed on display every 30 years (next in 1989).

Portions of this magnificent cathedral, including a wall, were actually Helena's personal palace, and other sections, mostly foundation stones, were part of a basilica begun by Constantine in 326. The cathedral is the epicenter of Trier's dazzling array of Romanesque and Gothic churches: It is adjoined, via the 13th-century cloisters, to the Liebfrauenkirche, the Church of Our Lady, built on 4th-century foundations, completed in 1260 and regarded as one of the chief examples of early Gothic style in Germany. The 12th-century Basilica of St. Mathew is based on a 9th-century structure and was completed in 1148. St. Maximus, reconstructed in 1680-98, is part of a former Benedictine monastery. And St. Gangolf's, the parish church, which towers over the market square, was consecrated in the 15th century.

The Synagogue, founded in the 11th century but no longer in use, is one of the oldest in central Europe.

The epoch of Trier's greatest splendor lasted until 395, the year in which Emperor Theodosius I died and the Roman state was formally divided--between his sons Arcadius and Honorius--into eastern and western empires. That division heralded the fall of the Western Empire and the advent of the Dark Ages, with Germanic tribes sacking Rome itself and ravaging Gaul, Italy and northern Spain. The Franks conquered and destroyed much of Trier and contributed to the city's decline. Only the bishopric remained, preserving the lamps of culture and civilization in a city that had become virtually a barbarian encampment.

It was not until after the division of Charlemagne's realm in the 9th century, when Trier and its hinterland became part of the territory of the Eastern Franks, which developed later into the Holy Roman German Empire, that the city experienced a revival. Much of the rebirth was due to the fact that the archbishops of Trier acquired temporal powers in addition to their ecclesiastical ones. It was under these prince-bishops, who were also among the seven electors of the Reich, that the city blossomed again and became an important medieval metropolis.

This era of Trier's rebirth is represented by the profusion of churches, chapels and monasteries and by a kaleidoscope of secular buildings and merchants' palaces around the market square and along Simeon Strasse, the main shopping street. Two of the loveliest are the Steipe, erected by burghers in the 15th century as a dance hall and community theater, and the "House of the Three Kings," so named because of the relief effigies of the Wise Men on the facade.

Incongruous as it may sound, for all its role as a font of early Christianity, Trier is also the wellspring of modern Socialism and Communism, for Karl Marx himself was born there in 1818, the son of a prosperous local lawyer, and spent his youth in the city until matriculating at the University of Bonn. The house in which he grew up, at Brucken Strasse No. 10, is now a museum and open daily, admission 35 cents.

Beyond history, architecture, art and monuments, Trier offers much in the way of gustatory delights. Situated in the heart of the Moselle-Saar-Ruwer wine district, it is virtually synonymous with the light, dry whites of that region. They ripen in the vineyards on the hills that surround the city, making it a kind of mecca for serious imbibers, and also gourmets. There are dozens of quaint taverns in the Olewig area, southeast of the center of town, and a number of vintners there offer wine-tasting feasts, accompanied by hearty meals of charcoal-broiled steaks.

One of the quaintest and most popular wine restaurants is "Zum Domstein," facing the market square, which has been the property of a family of vintners--Alfons and Rosemarie Gracher--for generations. The wines--a choice of 240--come from Gracher's own cellars; his wife presides over the kitchen. Among her specialties: eel steamed in Moselle Riesling with a dill-seed sauce, and a souffle' of Moselle pike.

For those with a penchant for elegant dining, the most sophisticated spot in town--haute cuisine in every respect--is the "Pfeffermu hle," in a renovated 18th-century fisherman's house on the banks of the Moselle. It rates a star in the Michelin Guide for Germany. Proprieters Siegbert and Angelica Walde--he presides in the kitchen, she in the dining room--are typical of many young West German couples who have discovered that eating and drinking entails more than merely stilling hunger and thirst, and that cooking is not a trade but an art. Among some of Walde's memorable offerings: escargots in red wine, Moselle salmon poached in a mousseline sauce, and tournedos of beef stuffed with goose liver.

Trier has an endearing way of living very much in the present and of putting its ancient monuments and relics to current use. Constantine's amazing palace, for example, does double duty as a Lutheran church. The amphitheater serves as a site for rock and pop festivals. Convents and monasteries are today used as hospitals, orphanages, old-age homes and as government office buildings. The "House of the Three Kings" on Simeon Street harbors the city's best cafe and pastry shop.

Unadulterated by the 19th-century Wilhelminian kitsch, and relatively unmarred by the prefab, steel-and-glass commercial architecture that has blighted many a West German town since World War II, Trier is small and unpretentious, but rich and lovely--a charming city that, if you believe the legend on the "Rotes Haus," was already old when Rome was being built.