The Château d'If was Marseille's answer to Alcatraz: a brooding island-fortress-prison which has played host to some extraordinary guests: victims of religious persecution, degenerate young roués, anti-royalists and revolutionaries and, most famously (and fictionally), Alexandre Dumas père's great romantic hero, the Count of Monte Cristo.
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The Château started life as a fortress, built in 1524-31 on the orders of King François I as a defence against attacks from the sea, and was instantly controversial. Marseille had been annexed to France in 1481, but the city retained in theory the right to provide its own defence.
The new Château seemed to many like an unwelcome reminder of royal authority. And although it successfully repelled an attack on the port by Charles V of Spain in 1536, as time went on the cannons proved inadequate to reach invading ships.
The Château became a prison in the mid-16th century. Among its first guests were a couple of fishermen and one Anselme, a knight accused of plotting against the monarchy who died, strangled, in his cell.
Subsequent inhabitants over the next 200 years included 3,500 Huguenots (French Protestants) who earned their keep as galley slaves and a Monsieur de Niozelles who was given six years for failing to take his hat off in the presence of King Louis XIV.
Others were imprisoned without trial under so-called lettres de cachet, supposedly signed by the King, for minor misdemeanours (a popular ploy used by moneyed families to get rid of unruly offspring without causing a public scandal).
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The writer and revolutionary Honoré Gabriel Riqueti, Comte de Mirabeau, a legendary ladies' man despite his startlingly ugly looks, did time there in 1774 and promptly seduced the woman who ran the canteen (his stint inside was to do him no harm: the main boulevard in Aix en Provence was later named after him). Gaston Crémieux, a leader of the Paris Commune was shot there in 1871.
But the Count of Monte Cristo remains the Château d'If's star prisoner, and the key reason for its continuing mystique and fascination. Dumas (pictured left) set his thrilling revenge adventure during the early decades of the 19th century and published it in 18 instalments between 1844 and 1846.
Very often adapted for film and television, it tells of Edmond Dantès, a sailor unjustly accused of treason on the eve of his wedding and holed up for 14 years before becoming the only inmate to escape by swimming across to the city. Today, his exploit inspires an annual swimming race, the Monte Cristo Challenge.
One of the island's most oddball denizens arrived years before the Château was commissioned. A rhinoceros had been presented by the King of Gujarat in India to the King of Portugal who, in turn, decided to pass it on to Pope Leo X.
The ship from Lisbon to Rome, bearing the beast splendidly kitted out in a "gilt-iron chain and a green velvet collar decorated with gilt roses and carnations", had to make a stop-over at If.
Here the rhino remained as an attraction for several weeks, inspiring the German artist Albrecht Dürer to produce a famous woodcut in 1515, pictured below (Dürer never actually saw the animal but worked from various second-hand descriptions). Alas, the ship bearing the rhino onward to Rome was wrecked en route and the animal was presented to the Pope as a stuffed trophy.
Dripping with history and brooding atmosphere, the Château d'If is itself an impressive beast and the boat ride across from Marseille makes for a delightful day trip. But little effort has been made to maximise its incredible story (the refurbishments of 2010/2011 apply only to the landing stage, and not the interior of the Château).
Many of the printed explanatory signs are in French only (though you are given quite an informative leaflet on the way in). A number of cells are simply shells, even if it's spookily fun to imagine oneself a prisoner there; some of the display cabinets are empty. There's a small display on the history of the rhino and an inevitable gift shop.
But you can tour through cells of different sizes, some of them - so-called pistoles which were rented at a premium by richer prisoners - relatively luxurious, with fireplaces and high ceilings. You can also climb up to the top of one of the towers for terrific 360 degree views of the bay.
A number of panels introduce the colourful figures of Dumas (exploring his mixed-race origins) as well his hero and a few of the rooms have screens playing key scenes from some of the many film adaptations of The Count of Monte Cristo.
If is one of four Frioul Islands which represent extreme examples of a Mediterranean habitat. Their fierce little micro-climate has been chiselled by the Mistral wind and blazing sun and they are one of the driest sites in France.
Consequently, they have developed some 350 rare species of plants, including varieties of samphire, sea lilies and sea lavender and are home to birds such as the bluebird, kestrel, peregrine falcon, little owl and yellow-legged gull, known locally as le gabian.
In July 2010, an experimental "underwater path" was set up just off Saint Estève beach on the island of Ratonneau to present its rich marine life. Five buoys roughly 25 metres apart mark five underwater panels explaining (in French) the flora and fauna to be found at each spot. Don't forget your snorkel, flippers and swimsuit if you want to try this out.
As Marseille developed as a port, the islands were used as a stopover for sailors and subsequently a quarantine station to protect the city from epidemics.
Ratonneau (pictured), the largest island, houses the Hôpital Caroline, built in the early 19th century as a quarantine station to treat people suffering from yellow fever.
It's currently being restored by teams of volunteers and, each year in July, provides a magical, open-air backdrop to the popular and long-established MIMI Festival of avant-garde music.
How to get there and get around: A regular boat shuttle leaves from the Old Port all the year round, weather permitting. It stops first at the Château d'If, then at Port Frioul on the Ile de Ratonneau. The journey takes about 20 minutes to If and a further10 minutes to Port Frioul before returning on a loop directly to the Old Port. Details of the Frioul boat shuttle. Note that this service may not run on days of high winds.
Road traffic is banned but, on Ratonneau, a petit train (little tourist train) runs from Port Frioul to the Hôpital Caroline in July and August only and tours by horse-drawn carriage are also available in summer.
Where to eat and drink: The snack bar and restaurant on the island of If is now closed indefinitely. Visitors are strongly advised to bring their own refreshments (at the least, water) as If can get extremely hot in the fierce midsummer sun.
On Ratonneau, there is a cluster of restaurants and brasseries on the harbour front and in summer the ferries to the mainland run late enough to have dinner there. At lunchtime, a more attractive option might be a picnic at Saint Estève beach or in one of the island's many delightful creeks.
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The Count of Monte Cristo (book and DVDs)