One of ancient Rome's most popular and important landmarks is "close to collapse", covered in graffiti, with valuable frescos peeling away.

Castel Sant'Angelo, whose parlous state was revealed yesterday by Corriere della Sera, one of Italy's most authoritative newspapers, was built by the Emperor Hadrian as his own mausoleum on the banks of the Tiber. Its proximity to the Vatican persuaded popes in the Middle Ages to add ramparts and battlements to the marble structure and use it as a shelter when the city was under attack. A passage between the castle and the Vatican, once used by popes in time of crisis, still exists.

Today it is almost as celebrated a symbol of Rome as the Colosseum, and it is the third most-visited landmark in the city. With interest in the ancient city certain to boom as a result of a new HBO/BBC drama series calledRome, set in the time of Julius Caesar and beginning this week, even more tourists are bound to tramp across Bernini's Ponte Sant'Angelo between the angels the sculptor called his "breezy maniacs", to the imposing citadel. But when they get there they are in for a nasty surprise.

The new drama series promises to give a warts-and-all vision of ancient Rome in contrast to the scrubbed marble and starched white toga tradition of the MGM epics. Rome in the new series, as The Independent's John Walsh wrote recently, "looks more like backstreet Tangiers or Calcutta, where the temples are dirty ... the streets full of mud and the walls covered in graffiti." Two thousands years on, Castel Sant'Angelo as it is today fits right in.

Neglect and bad management have reduced it to such a state of disrepair that there is reason to worry about its physical survival.

An air of seedy neglect hangs over the entire place.

Cobwebs drape the lamps, cigarette ends and pigeon droppings litter the ground. Beggars are encamped under plastic sheets in the dried-out moat. Within the castle, many rooms are closed to the public; but those that remain open look as if they should not be. Ancient frescos are crumbling away, as is the plaster on which they were painted, exhibits lack captions, ancient woodwork is rotting. At the entrance there is a scale model of the way the castle looked when first erected - but the model itself looks like another piece of abandoned rubbish, and its broken enclosure is repaired with sellotape.