His beard is now grey and thin. His speeches, once famed for their length and fire, are now less protracted and usually delivered from a chair. His climactic points are often spoken in a hoarse whisper, and there are long dramatic pauses. He employs gravitas where once he exercised passion. His gestures are slow and trembling. On the advice of his doctor, he no longer smokes cigars, although he sucks at his cheeks as if he did. When Fidel Castro seized power in January 1959, he overthrew a corrupt, ageing dictator and promoted a new generation of youthful visionaries. At that time it was Castro's youth that was an essential part of the appeal. He and his movement offered hope and change and regeneration – not to say revolution. As he turns 80, it is a measure of his considerable intransigence that he has grown old in office without holding a single free election in the intervening period.

As the world's longest serving head of state, he now cuts a rather different figure from the youthful revolutionary of old.

There are those who claim he is now little better than Fulgencio Batista, the dictator that he displaced. They claim that the only difference between the two is that Castro has brought equality: whereas under Batista a considerable proportion of the population were poor, under Castro everyone is equally poor – Cuba's GDP was higher than Italy's in the 1950s. It is a claim that would upset Castro's global supporters, of whom there are many. But at a time when he is passing another milestone, it is worth examining what exactly Castro stands for and the current state of Cuba.

One of the often overlooked reasons for Cuba's economic resilience is diasporic. Just as American Jews play such an important role in destiny of Israel, there is a large diaspora of Cubans (most of whom are in the United States) who pay large amounts of money to their relatives back home. It was estimated last year to be in the region of $1 billion per year: more than the revenues from the ailing sugar industry.

These sums are among the major reasons why Cubans manage to survive today in the crippling economic conditions of the post-Soviet era (in that oblique sense, the US is the biggest donor to Cuba). In addition, there is an active lobby of exiles in the United States that is rich and influential and tries to bend US policy its way (the Cuban lobby in the US modelled itself on the Israel lobby – with the important difference that the Cuban lobby is opposed to the current Havana government). Cuba is also a country whose cause the mainstream left once espoused, but has largely given up.

But also, again like Israel, Cuba is one of those countries whose politics has been gravely misconceived by forcing it into a left/right spectrum. One reason why Castro has survived for so long is that he has managed, through immense skill, to make it look as if he is a champion of the left and of the people while ignoring what one might assume were certain essential principles of the left, such as human rights and freedom of speech (although Castro archly attributes this to the aggressive policy of the United States and its embargo, which apparently make such restrictions necessary).

The attempt to understand this situation using a simplistic left/right axis is not only confusing, it obscures the picture considerably. To counterpose, for example, a right-wing neo-imperial US with a leftwing revolutionary Cuba – to suggest that they are implacable ideological opponents – is to take the propaganda at face value and miss the simple fact that Castro is the main beneficiary of the US embargo. He knows that his revolution would never survive an avalanche of American capital which would make many of his people richer, more independent and therefore more demanding. He acts with Machiavellian zeal to ensure that the embargo continues. When it looks as if the pressures are such that the American president might consider lifting it, he carries out some heavy-handed act, like the imprisonment of dissidents, or summary executions, that makes democrats cry off. Does this make him a figure of the left or the right?

Those that continue to believe in Castro do so because, in Europe at least, they are still relieved that a left (or what they believe to be the left) exists at all. And yet one sees signs of a rebirth of a different kind of left across Latin America, from Venezuela's Chavez and Bolivia's Evo Morales to Chile's Michelle Bachelet. Their governments are all democratically elected and, apart from the populism of Chavez, are, by and large, of the sensible left. Nonetheless, Castro's antagonism towards the United States is something that they all at some level approve, even if they would surely not commend the detail, and manage to maintain their own cordial relations with the United States regardless.

Until I went to Cuba for the first time, I too held a certain sympathy for an embattled island struggling to establish a socialist ideal in the teeth of US disapproval. The experience of visiting the place is, however, a powerful political lesson which makes one not only reject such utopias for all the cruelty, authoritarianism and fear that follow in their wake. It also makes one grasp just how misleading the categories of right and left can be in understanding such regimes.

Posing as a doyen of the left enables Castro to get away with things that a right-wing dictator would never have managed. The provision of healthcare and education are amongst Castro's achievements in Cuba – although the quality of both are sometimes exaggerated by officials, and even the UN statistics, from where people derive their argument, are submitted by Cuban officials rather than independent adjudicators as in other countries. (Foreigners who pay fees are usually treated far better in hospitals than ordinary Cubans, as I have personally witnessed).

But the broader point is that these provisions of healthcare and education by the state were also, for instance, things that the Nazis trumpeted in Germany. This is not to suggest equivalence, but only to point out that such achievements should not be seen as intrinsically of the left. Perhaps a better personal comparison is with the Spanish fascist dictator Francisco Franco. Both had family roots in Galicia in Northern Spain, both staunchly authoritarian, both socially conservative (Castro is, after all, famed for his punishing policies towards homosexuals). Both have strong tendencies toward megalomania.

But unlike Franco, Castro's regime has been forced to change direction in its twilight years. Castro has faced a dilemma over the last 16 years: should he let in the market or to keep it out? When the Soviet Union collapsed, many people would have assumed that Castro, dependent on Soviet support, could not survive. The subsidies that his Soviet sponsors had paid him petered out.

A period of intense difficulty ensued, dubbed 'the special period': a time when Cubans lived on remarkably little. Having institutionalised his revolution, the system over which he presided had rigidified. He was reluctant to allow the market free rein, even though everyone knew that people mostly survived this period because of the black economy (a version of the free market).

But he has survived thanks in part to the reinvigoration of the tourist trade (which has now practically replaced the sugar industry as the principal resource of the state), and to his new friendships in Latin America, especially with Hugo Chavez, who now provides sufficient oil to keep the Cuban revolution afloat.

Even with this much-needed assistance from Venezuela, the Cuban leader has had to wrestle with the idea of introducing a freer market. Private restaurants have been allowed, and then banned, and then allowed again but they must accommodate 12 chairs only (to ensure that they do not become too rich or successful, or too competitive with state-owned establishments); the dollar was legalised but the runners on the black market were pursued as the enemies of the revolution; economists were given some licence to draw up elaborate plans, but were then issued with warnings for veering too far from the revolution.

In the absence of Soviet sponsors to pay for his elaborate network of state provisions, Castro has recognised the need to harness the ingenuity and the muscle of the market, but he does not want to be ruled by it.

He has in many ways lost his revolutionary edge. Indeed his rule has been marked by extreme caution and indecision in recent years, rather than by transformative ideas. This may be understandable in view of the hostilities with the United States and the criticism from the exiles in Miami. But being a consummate politician above all else, he maximises the implications of these hostilities to the full. It is as a stalwart, embattled nationalist leader of a small island, standing up to an aggressive, neighbouring superpower, that he preserves his revolutionary credentials most effectively (his military fatigues seeming a curious piece of nostalgia for the Sierra Maestra nowadays). He knows too that he can capitalise on the latent anti-Americanism in Latin America, Europe and Canada – which has been on a sharp rise since September 11 – to make his struggle appear a universal one.

Castro's survival can be explained partly in these terms, but his own charisma is also a huge factor. He possesses an encyclopaedic knowledge with which he can often stagger his audience. He can talk about nail varnish and Swiss cheeses for hours with ambassadresses, or the fate of factories in Northern England with visiting businessmen. But his favourite subjects tend to be science and athletics, to which he brings a strange moral focus. It is all part of the inflated belief in himself as an Enlightenment figure.

Another factor in his allure is his penchant for paradox. His ability to shock and to make people sit up prevents him from being seen as one of those faceless, dried-up Eastern European leaders of old. Take the following. At a diplomatic reception in Havana, Castro was heard to talk about the current president of Galicia and General Franco's former information minister – in other words a significant pillar of the Right in Spain. Manuel Fraga was the only Western dignitary who was prepared to entertain Castro abroad in the first few years of the 1990s, partly because of the Galician connection. At the reception, Castro was describing how much he admired Fraga: "He is a good man", he said. "I would not even mind meeting Margaret Thatcher… in fact, if I had been a European, perhaps I too would have been on the right."

Whatever the international Left may think of Castro, ordinary Cubans have a rather jaundiced view. In the absence of free elections, it is of course difficult to gauge properly. One meets plenty of frustrated individuals who are incensed that they are allowed to do so little to improve their own livelihoods in the face of crippling poverty. In spite of some decentralisation, the state still controls almost every aspect of the economy, except where the black market gets away with it.

Those who are not lucky enough to have jobs in the tourist trade have to do inventive things like steal toothpaste or cigars from the factories where they work and sell them on the sly; or keep pigs on their balconies and sell the meat to neighbours in secret. The most frustrated even believe the free availability of health and education to be a form of blackmail, calculated to stall criticism. Such people can be intensely sarcastic in private about the 'comandante-en-jefe', but remain far more cautious with their remarks if they consider themselves to be with unknown company. But even such people have a highly ambiguous reaction to their leader. They are still awed in some degree, even if it is an awe that is tinged with fear.

It is unlikely that Castro will be ousted before he dies, but what comes afterwards depends to a large extent on the relative wrath that Cubans and Cuban exiles feel in the aftermath. On some reckoning, familial ties, financial dependencies forged during the special period and mutual curiosity will be too strong for Cuba's deep division to last far beyond the death of the man who symbolises it. But it may be many years yet until we know. His most restless and determined detractors have left the country, and Castro's most remarkable quality is his indefatigable staying power.

Isabella Thomas is a writer and journalist who has lived in Spain, Cuba and Mexico