Travel: There's no need to burn out in Bunsen's home town

Mark Twain tramped there, and Turner painted there. Margaret Campbell takes childhood images to Heidelberg

First impressions of a place can have strange origins: my picture of Heidelberg was formed through the viewfinder of a little plastic chalet, a souvenir brought back by family friends from their annual trip to Europe. Over the years, other layers were added to this childhood image of a ruined castle overlooking a river: the (rose-tinted?) memories of an older German friend who spent the Sixties there as a student; A Tramp Abroad, Mark Twain's account of his travels through Europe; David Lodge's portrait of post-war Heidelberg, divided between the victorious Americans and less prosperous locals, in Out of the Shelter.

Finally, years on from that first contact, I visited this celebrated town last month and realised again that some things are best experienced at first hand. Heidelberg really is breathtaking, particularly outside the summer season, when it is apparently overrun by tour buses and umbrella- waving guides. In any case, part of the city's magic derives from the mists that roll down over the hills, occasionally allowing a glimpse of their wooded banks, and from the traditional - and convivial - student presence.

There was no question about our first destination: we climbed the steep path to the castle, pausing regularly to wonder at the view beneath before passing through thick stone walls into the courtyard. It was hard to know where to look first: once home to the Palatine electors, the present structure has been added to many times since it was begun in the 15th century, and various architectural styles are juxtaposed with the ruins of an enormous fire in 1764. One of the most striking remains, an ornate Renaissance facade that serves as a backdrop to summer opera productions, reveals only sky behind the top two floors. The courtyard also houses an apothecary museum and the Heidelberger Fass, an enormous wine barrel made out of 130 oak trees in the 18th century and guarded by a statue of Perkeo, the heavy-drinking court jester who is reputed to have died after drinking a glass of water by mistake.

We could have spent hours exploring the castle interior and its museums, but were impatient to see the gardens and Elisabeth's Gate - an arched gateway allegedly built in a single night for James VI's daughter, who married Elector Friedrich in 1610. However, it was raining by now, so after a quick walk around, we visited one of Heidelberg's cafes, which almost deserve to be visited as attractions in their own right. A few slices of cheesecake later, and we were fit for more.

There has been a university here since 1386, making it Germany's oldest, and its buildings are scattered throughout the Alte Stadt. The most unusual has to be the Studentenkarzer, the dungeon for errant students (in the past, they were not subject to civil courts and could not be imprisoned in the town jail); the cell walls are covered in graffiti. The statue of Robert Bunsen brought back memories of school science labs, but this distinguished scientist was better known in his day for separating the colours of the spectrum.

Traditionally, the students were grouped into fraternities, with their own flags and rules of membership. Fencing skills were essential, and a fighting-scar was as much a rite of passage as a stay in the cells. The flags of the different fraternities, now existing more as optional social clubs, flutter over the streets. Student taverns abound, the most noteworthy of which is the Zum Roten Ochsen (Hauptstrasse 217), where Bismarck and Mark Twain both ate.

Much of Heidelberg was razed in the 1690s by invading French troops. One of the few buildings to remain standing was the ornate Haus zum Ritter, also on Hauptstrasse. This fanciful edifice, topped by a statue of St George and whose facade is covered in Latin inscriptions, served as the town hall before becoming an inn, still in business today. Opposite, the towering Gothic Heiliggeistkirche is a bit of a contrast. Once home to Germany's largest library, it was plundered during the Thirty Years War and its riches were transferred to the Vatican. Outside, the buttresses contain little market stalls, at one time common outside large churches. It was too cloudy to climb to the bell tower, but on a clear day you can see all the way down the Neckar valley and even make out the Alps in the distance. Marktplatz, between the church and the Rathaus, was formerly the site of public judgments, executions and humiliation (citizens could be placed in the Triller, a rotating cage, for minor offences up until 1740). And then we window-shopped along the (considerable) length of the pedestrianised Hauptstrasse and its narrow side-streets.

Next day, we crossed the river and walked up the Schlangenweg to Philosopher's Way, a charming walk that gives the best view of this apogee of German romanticism: steep-forested hillsides, a swift course of water and the castle, inspiration for Turner and described by Twain as "the Lear of inanimate nature". The winding path back down the valley took us out on the riverside, and we stopped for a few minutes to try to distinguish where the 18th-century ramparts of the Karl Theodor bridge (remember that chalet?) merged into the central pillars, reconstructed after the bridge had been damaged in the Second World War.

The Baroque towers and gateway leading back into the Old Town were also once used as cells. This constant reminder of criminality seemed at odds with Heidelberg's history as a centre for theology (Martin Luther came here to defend his doctrines, and the Heidelberg Catechism of 1563 became one of the textbooks of Calvinism).

And I even saw some plastic chalets for sale - but sent a suitably atmospheric postcard home instead.

From Britain, the most convenient airport for Heidelberg is Frankfurt, an hour's train journey away