Aachen, I can now inform you, is Germany’s most westerly city, nestling in the corner of the map where it meets Belgium and the Netherlands. I arrived there at about two in the afternoon, having travelled from London by a Eurostar train to Brussels-Midi, where I changed for an inter-city train to Aachen (or Aix-la-Chapelle as its called in French).

Its big selling-point, in guide book terms, is its connection with Charlemagn e, the 8th-century ruler of an empire that sprawled across north-west Europe. Aachen, formerly a Roman spa resort, was adopted as his capital, bringing the power of what became known as the Holy Roman Empire to this Rhineland town.

Although there is a prosperous, rather anonymous, post-war city centre, visitors to Aachen are likely to make for the old town, where shoals of daytrippers drift through a clutch of historic buildings and cobbled streets before settling into the handily-adjacent line of caf#233;s and restaurants. There’s an international feel to the place - you can hear German, French, Flemish and Dutch - and, catering for border-hopping visitors, many of the shops take Belgian and Dutch currency as well as Deutschmarks.

At the middle of this historic centre is the town hall (or Rathaus as the Germans so evocatively put it), a 14th-century structure, built over and incorporating parts of the much earlier royal palace of Charlemagne. This overlooks the town square, which you can identify by the traditional presence of a McDonald’s, Body Shop and a clapped-out hippie busker singing “American Pie”. A more modern aspect of streetlife in towns across western Germany are the gloomy-looking bands of East European musicians, singing and dancing for Deutschmarks.

The streets of the old town are comfortable and relaxed, rather than spectacular, so I consult my stack of books and pamphlets so I can make the most of my three hours here. As well as a strangely translated guide to the town warning me not to “make the buffoon” of myself at the casino, I find that all guide books point to two main sights - Charlemagn e’s cathedral (where he was buried in 814) and the Schatzkammer, which is a museum holding the cathedral’s treasure.

The cathedral, the core of which was built more than 1,100 years ago as a no-expenses-spared state-of-the-art showpiece, belongs to the era known to British history as “the Dark Ages”. Charlemagne’s architects were clearly unaware of this fact, as they designed a beautiful octagonal royal chapel that emulated the great churches of the Roman and Byzantine emperors. Aachen cathedral is one of the great links between the Roman empire and the medieval world, showing that while Britain might have lost its links with the classical past, in mainland Europe a strong sense of continuity remained.

I have to say that I’m not a great fan of cathedral treasuries, as you usually pay a few francs or lira or whatever to a leathery local and then find nothing inside except for a few dusty vestments and a collection of cryptic newspaper cuttings. But my guide book tells me that Aachen’s Schatzkammer is the finest cathedral treasury in northern Europe. Now, northern Europe is a large place, so with that kind of recommendation I had to pay a visit.

I wasn’t disappointed. In fact, when I found the separate museum building and paid my five Deutschmark s, I was dazzled. The Schatzkammer is the nearest thing I’ve seen to the children’s book image of a cave crammed with treasure - jewels, crowns, paintings, sculptures, icons, altarpieces, swords, sceptres.

Each piece seems to have a fabulous amount of detail and craftsmanship. The collection, with works of art from the Romans through to the Middle Ages, is held in a series of chambers that are kept in near darkness, with the only light coming from spotlights.

Like many truly beautiful things, there is a touch of strangeness about some of the artefacts. There is a larger than life-sized gold and silver bust of Charlemagne, made in the 14th century and supposedly holding part of his head, which stares fiercely into the darkness. There is a remarkable 10th-century gold cross, studded with ancient jewels and precious stones on one side, while on the reverse is a primitive, almost folk art engraving of the crucifixion.

Among all these lavish items, a beautifully simple 13th-century English sceptre, with a delicate gold bird perched on top holds my attention.

When I walk back out into the afternoon sunshine, I feel I’ve found something unexpected, the secret that makes Aachen “unmissable”. It makes the journey feel worthwhile, so that I can enjoy my walk back to the station, past the fountains, the dozens of bread shops and the offices of the local paper which has today’s issue on show in the window. I look at the lead story and see it’s illustrated by a large picture of a goat balancing on the back of a donkey. Yes, it really is summer.

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