When a boulevard in Palma de Mallorca was renamed to honour Princess Cristina de Borbón and her husband, Iñaki Urdangarín, in 1998, it seemed like a good fit. It was one of the city's most regal thoroughfares, lined with trees and dotted with fountains and Roman statues, and they were the feted new couple of the Spanish crown. The princess's father, King Juan Carlos, had recently given the pair the titles the Duke and Duchess of Palma.

Fifteen years later, the city changed its mind. Urdangarín was under investigation for embezzlement and questions were being asked about how much his wife knew. "He has conducted himself poorly and has shown a lack of consideration towards the title and the name of our city," said a spokesperson in explanation for why the city was reverting back to the old name of the boulevard.

On Saturday, just a few streets away from the renamed La Rambla, Spain's royal family will live out another chapter in their steady fall from grace. At 10am Princess Cristina de Borbón has been summoned to the city's imposing stone courthouse to answer allegations of money laundering and tax evasion.

The 48-year-old princess will be the first royal-born member of Spain's ruling Bourbons to appear in court since the monarchy was restored in 1975. She will be questioned by prosecutors, the investigating judge José Castro and dozens of lawyers over her role in one of Spain's longest-running corruption scandals. Both Cristina and her husband have denied any wrongdoing.

At its best, Cristina's appearance in court on Saturday could be cathartic for Spain's ailing monarchy, clearing her of wrongdoing and showing the world that justice plays out equally for every Spanish citizen, royal or not.

But at its worst, this weekend's court appearance could result in criminal charges against the princess, and up to six years of jail time and steep fines.

'The royal house is working to have her treated differently'

The gamble is not only Cristina's. What happens in this courtroom could also spell the end for Spain's King Juan Carlos. Once one of Europe's most beloved monarchs, the 76-year-old has seen his popularity plummet in recent years, leaving him fending off calls to abdicate and give the throne to his son, Crown Prince Felipe.

Hundreds of hours have gone into the logistics of this event. Letters and texts have flown between the court, police and the local government, with every new development tracked voraciously by Spanish media.

For security reasons, the princess will have the option to be chauffeured to the door of the entrance of the courtroom rather than walking down the 40-metre concrete ramp as most do. The front of the building will be cordoned off and protesters corralled away from the building. Saturday's questioning will take place behind closed doors and anyone who enters the courtroom will have their mobile phones, laptops and tablets confiscated. In a bid to protect the princess from images that could hurt her later on, the court will only record audio of the testimony, to be kept in a locked safe by the investigating judge.

The far-right anti-corruption group Manos Limpias has appealed against each of these decisions. "We're trying to have her treated like any other citizen of Spain," said Miguel Bernad. "But we're seeing that the power of the royal house is working to have her treated differently."

Examples they cite include Spain's anti-corruption prosecutor, who accused the investigating judge of "conspiracy theories" against the princess in a written statement and added that the magistrate had not accepted reports by Spain's tax agency that cleared the princess of wrongdoing. The Spanish prime minister, Mariano Rajoy, also recently said in a TV interview that he was "convinced of the innocence of the princess" and added that he was certain "things would go well for her".

'These sums were used strictly for personal spending'

The royal family has also been doing its best to sever ties with Urdangarín. The king banned him from official events shortly before he was named as a formal suspect in 2011. His official biography and most references to him were scrubbed from the royal family's website.

Even so, Cristina's name continued to come up in the investigation. Documents have surfaced that purportedly show the princess signed herself as owner and tenant of the couple's €6m mansion in Barcelona, raising questions about how the couple had paid for the house and the additional €3m worth of renovations they put into it.

The investigating judge José Castro. Photograph: Enrique Calvo/Reuters

The investigating judge's first attempt to get answers of out of Cristina failed to stick. Castro summoned the princess in April, but his demand was thrown out by a higher court in May, who said there was not enough evidence to warrant her testimony. Last month she was summoned again, this time for alleged tax fraud. Castro backed his request with a 227-page order, in which he noted it would be "difficult for Urdangarín to defraud Inland Revenue without his wife's knowledge and acquiescence". Cristina, he said, "chose to look the other way".

He cited myriad items bought by the princess – ranging from lavish trips and dinners to Harry Potter books – and paid for by a shell company the judge said was used to funnel money from her husband's non-profit institution.

"These sums were used strictly for personal spending … And they should have been declared in income statements … But it is evident that neither Iñaki Urdangarín nor Mrs Cristina de Borbón ever did so, which would suggest they repeatedly defrauded the tax authority," wrote Castro.

The summoning incensed the princess's lawyers, who vowed to appeal. Days later they backed down and said Cristina would "present herself voluntarily" to the court appearance she had been ordered to attend.

So far, Cristina's strategy has been one of "playing both cards at the same time", said the political commentator José Ignacio Torreblanca. She has tried to show that while she may not have known about her husband's activities, she remains loyal to him. "The princess has to decide between the house and the husband, and whether she wants to follow him to the end," he said.

Media leaks aside, the closed courtroom means the public will not get a sense of Cristina's testimony until the investigating judge publishes his summary further down the road, said Torreblanca. Instead, he believed, the act of showing up in court will speak volumes, giving some rest to the notion that the royal family is beyond the reach of Spain's justice system. "Once she testifies, no matter what she says it will be good for the image of the crown."

It might be a welcome change. A constant barrage of leaks to the media during the three-year-long investigation has so far only piled scandal on to the royal family, whether it was emails that divulged attempts by the king's associates to find employment for Urdangarín, or the crude emails sent by Urdangarín to his wife and friends, revealing a sense of humour that revelled in mocking women's intelligence and obscene jokes about his sister-in-law Letizia, who is married to the next in line to the throne, Crown Prince Felipe.

During the investigation, Cristina and her husband have shunned public appearances. Urdangarín remains in Spain, while his wife and their four children moved last year to Geneva where she works for the charitable foundation of a Spanish bank.

The courtroom in Palma de Mallorca where Princess Cristina is due to testify. Photograph: Enrique Calvo/Reuters

Their absence has not eased the pressure on the royal family. A recent poll by El Mundo found 43% of Spaniards would like to do away with the monarchy. Nearly two-thirds said King Juan Carlos, frail and weak after a series of operations, should abdicate. It is a stunning reversal of fortunes for the man once considered one of the world's most popular monarchs. Taking the throne just two days after the death of Franco in 1975, he won the respect of Spaniards by steering the country from dictatorship to democracy, foiling a coup attempt along the way in 1981.

But as Spain fell into financial crisis, the king's standing went into freefall. A low point came when just weeks after telling a reporter that he was so distraught about the growing ranks of the unemployed that he was having trouble sleeping, it was revealed that he had taken a luxurious trip to Botswana to hunt elephants.

Many began to see the king as part of Spain's problems, linking him to the economic and political powers who had driven the country into the crisis. This view of the monarchy is particularly prominent among young Spaniards, said the author of several books on the royal family, José Apezarena. "The history of the monarchy is unknown to them. They only have the few photos that come on television and the reports."

The consequences of this perception could be grave, given the royal family's tenuous grip on power. No reigning Spanish king has managed to pass on the crown to his offspring since 1885.

As Spain's royals find themselves under increased scrutiny, attempts are being made to show they are up to the challenge. In the past few months, they have launched a website explaining the Spanish crown to youngsters, announced pay cuts and gone public with each of their salaries. The royal family now wants to be a "glass house" when it comes to accounting for the €7.78m they will take from the public purse this year, said a spokesperson.

It is all part their new motto, say sources close to the royal family. Deep in the throes of their ongoing crisis, they are taking a line from the Spanish Nobel-prize-winning author Camilo José Cela to heart: "He who resists wins."

The case

Iñaki Urdangarín, a former Olympic-medal-winning handball player, is under formal investigation for alleged fraud, tax evasion, falsifying documents and siphoning off €5.8m in public funds through his charitable foundation, the Nóos Institute. Under the banner of organising sports and tourism conferences and providing consultancy advice, it is alleged Urdangarín and his former business partner Diego Torres used their connections to win public contracts, overcharged for services and hid the money in privately owned companies and offshore tax havens.

The money, allege investigators, was laundered through the Nóos Institute and Aizoon, where Cristina was a board member and co-owner, respectively. Investigators suspect the princess was aware of at least some of her husband's alleged crimes.