Forty miles outside Madrid, among the pine-spiked peaks of the Sierra de Guadarrama, sits Spain’s most famous Rorschach test.

Many look at the concrete and granite contours of the Valley of the Fallen and see a mass grave masquerading as a memorial; a fascist temple that continues to exalt Gen Francisco Franco and his 36-year dictatorship.

Others claim the basilica and its enormous cross are a genuine monument to national reconciliation intended to honour all the civil war dead.

Then there are those who view it as a haunted bunker still roiled with the ghosts of Spain’s dictatorial past. The socialist government, they say, should drop its plans to exhume Franco from his crypt in the valley and leave the past in the past.

Nicolás Sánchez-Albornoz, 92, a historian, has his own thoughts on the site, which he always refers to as Cuelgamuros, after the valley where it was built. For him, the Valley of the Fallen is a monumental fiction.

Nicolás Sánchez-Albornoz. Photograph: Collect

In March 1948, Sánchez-Albornoz arrived in Cuelgamuros on the back of a truck to serve out a six-year sentence for posting propaganda against the Franco regime around Madrid’s Central University.

“The Valley of the Fallen is a later invention – from the 1950s,” he says. “It was just the Cuelgamuros monastery detachment when I was there. Back then it was about building a massive church deep in the mountain. It was a religious exultation of the result of the civil war; no one talked about Franco or burials.”

By the time he arrived, the basic building work on the monument had been completed – partly by political prisoners used as forced labour – and Sánchez-Albornoz was given an office typing job.

“I’d already been in two prisons, and things in Cuelgamuros were better,” he says. “It’s a beautiful place: pine forests beat being in a prison cell and so it was a step up.”

But, he adds: “It wasn’t so pleasant as to stop me wanting to get out of there.”

Sick of the exploitation of political prisoners, and horrified at the prospect of having to spend two years in a military works battalion in Morocco once his original sentence was completed, he started planning his escape with the help of a student network in Paris.

On 8 August 1948, Sánchez-Albornoz and his friend, Manuel Lamana, crept away from the camp and stole through the woods to the gates of the nearby royal monastery of El Escorial. There they were met by two young American women, Barbara Probst and Barbara Mailer, sister of the writer Norman Mailer.

In a tiny Renault that the American author had bought on a recent trip to Europe, the four managed to escape to the French border, relying on real and false documents, the bravery of the women and a hefty dose of luck.

Sánchez-Albornoz may have escaped Cuelgamuros – and, in doing so, pulled off a remarkable propaganda coup against the Franco regime – but his long exile had only begun.

He spent the following decades in Argentina and the US, returning to Spain for good only in 1991 when he was appointed the first head of the Instituto Cervantes.

In the meantime, Franco remained in his crypt in the valley, interred since his death in 1975, undisturbed and even quietly celebrated.

For all the talk of post-war reconciliation, and despite the fact that more than 30,000 people from both sides of the conflict are buried in the basilica, only two graves are marked. One is Franco’s; the other is that of José Antonio Primo de Rivera, the founder of the Falange party.

More bodies were needed after the Franco regime decided to rethink the basilica and pass it off as a shrine to both nationalist and republican dead. And so thousands of corpses were exhumed from cemeteries and graves across Spain, often without permission from the families.

“People say there are fighters from both sides buried there, but that’s not true,” says Sánchez-Albornoz. Most of the republicans interred in the Valley were people who had been put up against walls and shot, he says.

“There are fighters from an army that rose up – and there are people who were shot, who were victims. They were victims. It’s a fiction that they wanted it to be a Valley of the Fallen.”

Ultimately, he says, there is “no parity between the two groups of people buried there”.

The government’s efforts to remove Franco from the valley and so close wounds “that have been open for many years” are proving far more difficult than originally thought.

While the state can order the exhumation, only the dictator’s family can determine where he should be reburied. If he is to be laid to rest once again, the family wants it to be in their crypt in Almudena Cathedral, in the centre of Madrid.

Mindful that such a move would only make Franco’s tomb more accessible to those who still celebrate his dictatorship, the government has sought the help of the Vatican and said it would use the law to stop a reburial in the cathedral.

Sánchez-Albornoz acknowledges there is no easy solution to “the problem of the Valley” but argues that the issue is about the monument as well as the man.

While the government makes up its mind, he says, it should stop all state funding to the Valley of the Fallen and instead let the grass grow and the wind blow.

“The monument is in bad shape. It’s on a mountainside, exposed to the elements, but it was also built from poor materials because the firms who built it were stealing – just like firms still do today.

“The wind and the rain are dissolving the statues. They shouldn’t spend another cent on the site. They should let nature take its course while they look for another solution. The sooner Cuelgamuros disappears, the better.”

Although Cuelgamuros features heavily in his memoirs – which bear the self-explanatory title Prisons and Exiles – Sánchez-Albornoz has never been back. “I’ve settled my accounts with that place,” he says.

“Four months was quite enough and I have neither the curiosity nor the bitterness to go back. I think those are the two things that tend to drive former prisoners back there.”

And anyway, he adds, raising his thumb to his nose: “I have the satisfaction of being one of the few people who managed to get one over on Franco.”