Sitting in a bar off the former fruit market in Prague’s old quarter, just by the Estates Theatre, where Mozart premiered several of his operas, I watch a pair of lovers sipping beer as they order oysters.

They could be Czech, or possibly American or Russian or Brazilian, but I know they’re lovers by their utter disregard for the sumptuous architecture all around them – and by where she has placed her hand on his jeans.

I watch to see if, as Giacomo Casanova once advised, they eat the oysters off each other’s lips. They don’t: they’re American.

Casanova walked these streets in the 1780s, sometimes with Mozart’s librettist, Lorenzo da Ponte, often with Count Waldstein and the Prince de Ligne, the old roué’s sponsors in his final years, spent exiled from Venice and writing his voluminous memoirs in Bohemia. Perhaps Casanova watched young lovers in the fruit market and reminisced.

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The backdrop to countless films – Amadeus, Kafka, The Unbearable Lightness of Being – Prague speaks eloquently of the passions of past generations. It is full of romance and a spirit of partying that has defied passing empires and ideologies. And, despite having one of the best-preserved baroque cityscapes in the world, this is defiantly not a museum.

Since the Velvet Revolution of 1989, it has rediscovered its past as the pleasure capital of Europe, the one Casanova knew so well. As one Czech proverb has it, there is always a party going on somewhere in Prague.

The Estates Theatre, only slightly remodelled since Casanova was a regular in its boxes, is where Mozart mounted the first production of Don Giovanni in 1787. Casanova was in the audience. He had good reason to be there: not only had he collaborated on the libretto, he’d practically lived the plot. Don Giovanni, an “update” of the tale of the legendary lover Don Juan, was partly Casanova’s story.

These days, the Estates hosts part of the city’s annual Mozart Festival – and frequent revivals of operas written for its stage, including Don Giovanni and La Clemenza di Tito – and offers tours that give real insights into the machinations of 18th-century theatre-going and opera production.

Creakily wooden, the opera house smells of polish and velvet, like the inside of an old violin case. The design plans for a rather special event now line the corridors of the boxes: it was held in 1791, when Casanova was here again, for a vast dance held in honour of the new Austrian emperor, Marie Antoinette’s little brother Leopold. The whole back wall of the theatre was demolished so that, as Casanova entered, the stage opened onto a ballroom that apparently disappeared into infinity.

By coincidence, Prague is also home to the vast Casanova archive – his letters, books, poems and mathematical treatises – found at Duchcov Castle, two hours by train from the city, where he died 210 years ago. Despite the mass of material, the extent of Casanova’s involvement with Don Giovanni is uncertain. All we have is several pages of his scribbled notes amending Act II, and a story told by an old Prague musician. It seems there was a picnic at Bertramka, home of Mozart’s hosts, Frantisek and Josefina Dusek, just outside Prague, and it was here that Casanova supposedly met Mozart. The house is still open to the public, and there are regular chamber concerts on the terrace.

Da Ponte arrived in town a few days later, obliged to rewrite a libretto with the added colour Casanova had inspired. The Prague audiences adored it. “It was given nightly for four weeks,” wrote Goethe, “the whole city being so excited by it that the merest shopkeeper had to perch in the stalls or a box with his whole extended family, and no one could bear to have missed seeing Don Juan roast in hell.”

Like Casanova, you may need to visit Prague many times to really get under its skin. The medieval Charles Bridge, cathedral and castle tend to dominate a first visit, but take a few steps sideways from the path over the Vltava River and you plunge straight into the 17th and 18th centuries.

Baroque buildings flaunt their bosomy charms in all directions and Prague’s churches, in particular, are alive to an almost constant music festival. There are concerts, recitals, sumptuously sung Masses, all of a remarkably high standard: I counted a choice of 24 events in one weekend within walking distance of my hotel.

Especially notable for both its acoustic and aesthetic appeal is St Nicholas’s, halfway up to the castle in the Mala Strana quarter, with a cherub on every available surface and a concert every day. In Casanova’s time, this was Prague’s latest baroque wonder, the finest work of master builders Krystof and Kilian Ignaz Dientzenhofer. Mozart himself played the organ here in the same year he wrote Don Giovanni.

For all the wealth of music that rings round the city today, the only Don Giovanni reliably in rep is the one performed by giant marionettes at the National Marionette Theatre, on Zatecka Street. For some, there’s an eeriness to the wooden puppets, but when I was there, the children in the audience lapped it up – a naughty hero getting a ghastly comeuppance and some very adult themes. More rarely in repertoire is a lavish reworking of the opera at the Estates Theatre itself, with the whole theatre recreated in mirror image on stage, Later, I found the truth in that proverb that a party is constantly buzzing somewhere in Prague, as I stumbled on a rock’n’roll night straight out of the 1960s. It wasn’t until I’d accepted my first dance and tried to pay for a beer that I realised it was a private party. Nobody seemed to mind.

A stranger in Prague, as Casanova once noted, is not a stranger for long.

Ian Kelly travelled as a guest of British Airways Holidays. His new biography of Giacomo Casanova (£20) is published by Hodder & Stoughton

TRAVEL BRIEF

Getting there: airlines flying to Prague include British Airways (0844 493 0787, www.ba.com), Czech Airlines (0871 663 3747, www.czechairlines.co.uk), EasyJet (www.easyjet.com) and BMI Baby (www.bmibaby.com ).

Where to stay: the Mandarin Oriental (00 420-233 088888, www.mandarinoriental.com) has doubles from £251. Or try the boutique Domus Henrici (220 511369, www.domus-henrici.cz; doubles from £95).

Tour operator: British Airways Holidays (0844 493 0758, www.ba.com/holidays) has three nights at the Mandarin Oriental from £679pp, B&B, including BA flights from Heathrow; three-star packages start at £253pp. Or try Great Escapes (0845 330 2084, www.greatescapes.co.uk).

Casanova spots: the Estates Theatre (224 224351, www.estatestheatre.cz; tickets from £52) has performances of Don Giovanni from July 9 until August 24. The National Marionette Theatre is near Old Town Square (www.mozart.cz). Bertramka (www.bertramka.com), where Mozart is reputed to have met Casanova, has a Mozart museum and stages concerts.

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