Clock-watchers, take note: The Sagrada Família has entered the home stretch of construction. And it took only 133 years.

Six new towers will soon be added to the (in)famous Roman Catholic basilica in Barcelona, bringing the total to 18 and—at long last—finishing the work begun by Catalan architect Antoni Gaudí in the late 19th century.

The tallest of the new towers will be 564 feet (172 meters) high, making the cathedral the tallest religious structure in Europe, says Jordi Faulí, the current chief architect. The building is now 70 percent complete and on track to be finished in 2026—the centenary of Gaudí’s death—though some decorative elements could take up to six additional years to complete.

Consecrated in 2010 by Pope Benedict XVI, the Sagrada Família remains an extravagant work in progress—a fever dream of deliquescent spires and vivid stained glass, ornate facades, and ornamental arches. Rising hundreds of feet above downtown Barcelona, it draws the eyes (and euros) of some three million visitors a year.

It’s impossible to say how much money the prolonged construction has cost over the years. Today the annual budget is reportedly $27 million, paid for partially by visitor entrance fees and private donations.

More quantifiable is the time it’s taken to build the cathedral. When asked why the project was taking so long, the pious Gaudí was fond of saying, “My client is not in a hurry.” He was talking about God.

When the architect died in a trolley accident in 1926, only one façade—and less than a quarter of the exterior—was complete. Since then construction has been waylaid by everything from protests to politics, civil wars to funding woes.

The Long Timeline

Born in 1852 near the town of Reus, Gaudí grew up fascinated by geometry and the natural wonders of the Catalonian countryside. After studying architecture in school, he eventually forged his own style—a sui generis synthesis of neo-Gothic, art nouveau, and Eastern elements.

For Gaudí, form and function were inseparable: One found aesthetic beauty only after seeking structural efficiency, which rules the natural world. “Nothing is art,” he concluded, “if it does not come from nature.”

In 1883, Gaudí inherited the Sagrada Família from another architect, who had laid a traditional neo-Gothic base. Gaudí envisioned a soaring visual narrative of Christ's life, but knew that the massive project could not be completed in his lifetime.

So for more than 12 years prior to his death, he rendered his plans as three-dimensional models rather than as conventional drawings. Though many were destroyed by vandals during the Spanish Civil War, those geometric models have been vital to Gaudí's successors.

“They contain the entire building's structural DNA,” says Mark Burry, an Australia-based architect who has worked on the Sagrada Família for 36 years, using drawings and computer technology to help translate Gaudí's designs for today's craftsmen. “You can extract the architectural whole even from fragments. The models are how Gaudí met the architect's challenge: taking a complex, holistic idea and explicating it so others can understand and continue it after your death.”

A Natural Model

The basilica has always been controversial—revered by some, reviled by others. The Surrealists claimed Gaudí as one of their own, while George Orwell called the church “one of the most hideous buildings in the world.”

As idiosyncratic as Gaudí himself, the basilica is a vision inspired by the architect's religious faith and love of nature. He understood that the natural world is rife with curved forms, not straight lines. And he noticed that natural construction tends to favor sinewy materials such as wood, muscle, and tendon.

With these organic models in mind, Gaudí based his buildings on a simple, syllogistic premise: If nature is the work of God, and if architectural forms are derived from nature, then the best way to honor God is to design buildings based on his work.

As the Barcelona scholar Joan Bassegoda Nonell says, “Gaudí's famous phrase, ‘Originality is returning to the origin,’ means that the origin of all things is nature, created by God.” Gaudí's faith was his own. But his belief in the beautiful efficiency of natural engineering clearly anticipated the modern science of biomimetics.

Prescient Design

Adrian Bejan, a professor of mechanical engineering at Duke University, says the façades of the Sagrada Família are based on the golden ratio—the geometric proportion “behind all aesthetically pleasing art.”

Bejan, whose "constructal law" states that design in nature is a universal phenomenon of physics, calls Gaudí a forebear and a “tightrope walker on the line bridging art and science. He understood that nature is constructed by laws of mathematics. What is strongest is inherently lightest and most efficient—and therefore most beautiful.”

At the heart of Gaudí's vision is a timeless truth. As Bassegoda writes: “Looking toward the future, the lesson of Gaudí is not to copy his solutions but rather to look at nature for inspiration … nature does not go out of fashion.”