Through the darkness, as you stand on the terrace along the walls of the great castle ruin, you see the glittering city below at the foot of Königstuhl Mountain. And you hear it, too - the ringing of church bells, the swishing of cars, the shouts of partygoers.
Your eyes can follow the long and gently curving Hauptstrasse, the main street, lit by brightly colored shops and by giant Christmas ornaments in the forms of snowflakes.
But the big event - Heidelberg's bustling Christmas market - is mostly hidden from view amid the crowded roofs of this ancient city.
But it's not far away. There are several ways to descend from the castle into town. None require more than a five- or 10-minute drive or walk.
Walk down the steps next to the castle, and you pass gracious old homes and elegant fraternity houses (linked to famous Heidelberg University, the country's oldest) that cling to the mountain. At viewpoints along the steep steps, wooded areas resemble rustic scenes depicted on Christmas cards. You feel as though you are in the 18th century.
When you reach the bottom, you are only a short distance from Heidelberg's three largest Christmas-market areas, in squares that are all adjacent to Hauptstrasse. (Some years ago, most motor traffic was banned from the mile-long Hauptstrasse, which was then given over to pedestrians.)
You sometimes can hear the Christmas markets - and smell them - before you even see them. A loud and colorful carousel provides musical accompaniment for the bustling activities in one of the market areas. Meanwhile, evocative drifts of smoke waft about, filled with news that some German wurst is on the grill.
Turn the corner, and there before you are the rustic wooden booths of dozens of merchants. Imaginatively adorned with lights and Christmas decorations, the booths have a quaint Old World feel.
The scene is at once modest and overwhelming. Each little shop is filled with colorful goods that seem to be clamoring for a visitor's attention. They may be as traditional as the simple wood carvings of Christmas scenes in one booth and as modern as the colorful toy figures at a booth nearby.
At one booth, the proprietor pumps traditional tunes on a hurdy-gurdy when not snatching up hard-to-reach ornaments for visitors. Here is where marketgoers find elegant grandfather-clock ornaments for their trees or an owl figurine to replace one that may have fallen prey to the paws of an inquiring cat. In common with other booths at the market, there is a sense here of quirkiness and old-time craftsmanship.
Another stall displays a village of miniature half-timbered ceramic homes, individually lit by candles. Nearby is an offering of sturdy woolen sweaters, scarves, and hats - winter wear such as an aunt might have knitted when you were a child, clothes you'd only appreciate as an adult.
The most lively booths often are those that sell pastries, marzipan, chocolates, and other sweets. Clusters of big, elaborately decorated gingerbread hearts with phrases like "I love you" hang from the ceiling. And the goodies themselves - the variety, shapes, and colors - convince you that one tummy is not enough to hold as much as you're tempted to try.
Fruity crepes, wooden toys, jewelry, stuffed animals, leather goods, glassware: It's all here, bursting out of the booths. You can buy carrots at one booth to feed the big-eared donkeys next door - a popular pastime with little girls, who enjoy admiring and fussing over the furry animals.
Some booths are run by local merchants, but many originate outside Heidelberg, and, in at least one case, outside Germany. At the Australian booth, you can take your pick of didgeridoos and other outback specialties.
There is a long tradition of Christmas markets in German cities such as Nuremberg and Munich - but, surprisingly, not here.
Heidelberg, with its glorious old buildings and location at the start of the Odenwald Mountains alongside the Neckar River, is one of the most beautiful cities in Germany, and, it would seem, a natural setting for a Christmas market. But the tradition began here only 20 years ago.
According to Inge Geis, secretary to the director of the Convention and Visitors' Bureau, the market was started because, for local merchants, December was a quiet time of year.
It's still quiet, relatively speaking, since many of the approximately 3 million visitors who come to see Heidelberg and its historic castle, its oldest-in-Germany university, and its churches, shops, and museums, crowd in during the rest of the year.
But the market is growing and becoming better known. Mrs. Geis has always run the Christmas market. "We started with 37 wooden houses in University Square ... and now we have around 130," she says.
The booths in University Square serve as colorful counterpoints to the elegant university buildings that form part of the square and fill some of the nearby streets. The University Museum and University Library are here, while many of the city's inspiring churches and museums are close by.
Market Square is dominated by the city's mighty Holy Ghost church. For about 50 cents, you can climb to the top of its tower for an exhilarating 360-degree view of the activity below.
At Christmastime, Heidelberg is filled with music, just as it is during other times of year. Some evenings, you can choose among several programs that take place in churches or at the university.
Last year, I attended a program of Schubert in the university's beautiful Alte Aula (old assembly hall) and one of the organ concerts at the Holy Ghost Church.
During Christmas market-time, Heidelberg hotel rooms, usually in short supply, are more plentiful. And it's possible to walk at dusk along Philosopher's Way - the appropriately named footpath along the opposite side of the Neckar - and meet practically no one. Weekends, however, are much more crowded, as locals gather to enjoy the market.
To be sure, December weather in Germany can be chilly and overcast. But for hearty travelers, such discomforts merely provide a richer real-life experience, one that will be remembered for a very long time.
This year, Heidelberg's Christmas market will be held from Nov. 28 to Dec. 23.
For the quintessential German winter experience, a visit to one of the country's numerous Christmas markets is a must. Vendors sell traditional German gifts, including handcrafted ornaments and wooden toys. It's hard to resist the aroma of fresh gingerbread as it wafts through the air.
These are popular Christmas markets:
Augsburg (Dec. 1 to 24)
One of Germany's most beautiful Christmas markets, set against the city's Renaissance town hall.
Dresden (now through Dec. 24)
Famous for its Dresdener stollen, a fruit-filled holiday yeast bread.
Dinkelsbühl (Nov. 29 to Dec. 23)
A gingerbread-like town, lined with cobblestone streets, makes this fair a favorite.
Lübeck (now through Dec. 12)
Renowned as the home of the world's best marzipan.
Nuremberg (Nov. 30 to Dec. 24)
A festive opening ceremony includes costumes and holiday music.
Regensburg (Nov. 30 to Dec. 23)
Glassware, ceramics, wooden toys, silk painting, and puppets line the Lucretia Market.
Rothenburg (Nov. 30 to Dec. 22)
The famous Christmas store, Kathe Wohlfahrt, is nestled in this medieval walled town.
Würzburg (Nov. 30 to Dec. 23):
Along with the usual market, there's a special artists' market in the town-hall courtyard.
Source: The German National Tourist Office