Madrid is set to unveil what is touted as one of Europe's cultural highlights of the year with the opening in the Spanish capital of The Royal Collections Gallery, adding to its long list of renowned galleries that it already boasts, like the Prado Museum, the Thyssen-Bornemisza, and the Reina Sofia.

The swanky new museum, the latest of Madrid's world-ranking art establishments, will feature master paintings, tapestries, sculptures, decorative art pieces, an armory and sumptuous royal furniture collected by Spanish monarchs over five centuries, spanning the empire's Hapsburg and Bourbon dynasties.

"This is the biggest museum project in Spain in decades, and also in Europe," says Ana de la Cueva, President of the Patrimonio Nacional. This government body runs the Gallery.

Unlike many other monarchies, Spain's Royal Collections belong not to the Crown but to the public, thanks to a historical twist nearly a century ago. Now, Patrimonio Nacional oversees palaces, monasteries, convents, and royal gardens across the country.

For Gallery director Leticia Ruiz, bringing together such a variety of extraordinary pieces makes it something of "a museum of museums."

The inaugural exhibition will feature 650 of the more than 150,000 pieces Patrimonio Nacional manages, including works from Velázquez, Goya, Caravaggio, Titian and Tintoretto. Also featured will be some pieces from the world's best tapestry collection, ancient carriages and royal furniture. A third of the works will be replaced with new exhibitions each year.

Ruiz says the Gallery will offer visitors a unique vantage point of "the history of the Royal Palaces that are fundamental to the history of Spain and the world."

One standout piece is Velazquez's "White Horse," rearing up without a rider, suggesting the court painter was just waiting to be told which king to put in the saddle.

Nearby, the light and facial expressions in Caravaggio's 1607 "Salome with the Head of John the Baptist" are equally captivating. The painting is one of just four Caravaggios in Spain.

Then there is the multicolored cedar wood sculpture of Saint Michael Slaying the Devil, a 1692 work by Spain's first female court sculptor Luisa Roldán. It is known that she carved the devil in the likeness of her husband and that she, herself, may have been the model for Michael.

On the same floor is the first edition of Cervantes' "Don Quijote."

"For many centuries, the Spanish monarchs were the best collectors in history," said De la Cueva. Being able to buy and order from the best artists in the world "was a way of showing their power."

Built on the steep hillside opposite Madrid's Royal Palace and the Almudena Cathedral, the Gallery building is an impressive artwork.

Designed by Luis Mansilla and Emilio Tuñón, its unimposing vertical linear structure has won 10 architectural awards, including the 2017 American Architecture Prize.

Unseen from street level, it descends seven floors. In the Hapsburg rooms, you are greeted by four gigantic baroque Solomonic faux marble wooden columns with gilded vines that once belonged to a Madrid church.

What makes the Gallery particularly special is its incorporation of Madrid's ninth-century Islamic foundation after archeologists came across part of the city's Moorish wall during construction.

Madrid was originally called Mayrit in Arabic and its Islamic rulers built a fortress to protect the nearby center of power, Toledo. Following the reconquest of Spain by the Catholic monarchs, Madrid was converted into Spain's royal court and capital in 1561 by Felipe II.

Alvaro Soler Del Campo, archaeologist and chief curator of the Royal Armory says Madrid "is the only current capital of the European Union that preserves a fragment of its first (founding) walls" as well as being the only European capital city that has Islamic origins.

The initial idea of building a museum to house the Crown's collections arose during Spain's anti-monarchy Second Republic between 1931 and 1939. Then, the leftist government seized the royal properties but protected them under a new agency that preceded the Patrimonio Nacional.

The republic was flattened during a rebellion by late dictator Gen. Francisco Franco and other Catholic Nationalist officers that started the three-year Spanish Civil War and heralded some four decades of dictatorship at its end in 1939.

Two decades after Franco's death and the return to democracy, the initiative for a museum was taken up again in 1998. But it took another 25 years, 172 million euros ($186 million), and several government changes before the ambitious project could be finished.

Ruiz says the novelty of seeing such artistic beauty in such a modernist building will appeal to visitors.

"What we want to do is capture them as soon as they enter, and I think we are going to do that," she said.

King Felipe VI and Queen Letizia will inaugurate the Gallery on June 28, after which it will be open to the public for the first few days.