Visit the fashionable La Casa Encendida gallery in Madrid and you can see the evidence. Officially, the Spanish civil war ended in 1939, but as the acclaimed novelist Javier Cercas puts it when we meet in his Barcelona studio: 'This is the past that has not passed. The nervousness that surrounds the monument, an hour or so outside Madrid, is proof of the long shadow still cast by the event that defined 20th-century Spain: the civil war that began 75 years ago, when Franco mounted an army rebellion against the democratically elected government of the republic.

Should there, for example, be an exhibition detailing the monument's history, explaining that much of it was built by up to 20,000 prisoners drawn from those Franco had defeated? The government is considering legislation to deal with the Valle de los Caídos, appointing a panel of historians to advise. To be safe, on 20 November – the anniversary of the dictator's death in 1975 – visitors' car boots are checked to ensure they are not bringing in placards or flags associated with Franco's fascist Falange party. Or perhaps by the plaque that honours Franco as the 'caudillo de España' (military leader of Spain), who built the Valley of the Fallen in 1959, or by the sight of the Franco crest on the plaza walls outside. But most believe the government wants to ward off those who might be drawn by the two words, etched into the church floor on the far side of the altar, accompanied by a simple cross, Francisco Franco: a burial spot marked on the day I visited by two bouquets, one made up of blood-red roses. The official explanation is restoration work. The result is that, on this sleeting March morning, the priests outnumber the congregation, with no more than 10 people in the pews. Just to make sure, a guard hands arrivals a slip of paper, reminding them that they are there 'for religious purposes only'. Since 2009, the site has been closed to general visitors only those attending mass are allowed. Indeed, much energy has been devoted to taming it.

Spain's socialist government understands the potency of Valle de los Caídos, or Valley of the Fallen. There is no denying the sheer, intimidating power of both spectacle and location: the Catholic-raised translator at my side gasps, confessing that he has never seen anything like it. An altar boy rings a bell and, at that second, the lights are turned off, filling this cavernous place with darkness – save for the beam shining on Christ upon the cross. At the altar 14 purple-clad Benedictine monks conduct morning mass, a solemn ceremony faithful to the most unchanged Catholic ritual.

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