From the time Romania's Stephen the Great took power in 1457 until his death in 1504, he fought 36 battles against the mighty Ottoman Empire, winning all but two. And the warrior king was as religious as he was ferocious. He built churches after many victories, filling the tiny enclave of Bukovina—part of the ancient principality of Moldavia—with Orthodox sanctuaries and monasteries. He was made a saint in 1992, and last year, a Romanian national television survey named him the greatest Romanian in history.

But it was Stephen's illegitimate son Petru Rares who turned his father's church building into something extraordinary. Beginning around 1530, Petru Rares, who ruled from 1527-1538 and again from 1541-1546, promoted a new vision for Bukovina's churches. He commissioned artists to cover the interiors and exteriors of a number of structures with elaborate frescoes (portraits of saints and prophets, scenes from the life of Jesus, images of angels and demons, heaven and hell) and encouraged his nobles to decorate their local churches in the same manner. The spectacular results are still in evidence today, nestled in the wooded valleys of northeastern Romania. The best preserved are part of rural monasteries in the towns of Sucevita, Moldovita, Voronet, Humor, Suceava, Patrauti and Probota. Another, a small church, is in the village of Arbore. Seven of the churches were placed on Unesco's World Heritage list in 1993, and the Romanian government is pressing to have the eighth, Sucevita, one of the last to be built, added soon.

Petru Rares and his spiritual advisers hoped to bring the lessons of the Bible to priests and illiterate peasants alike. "This was a way to communicate without words," says Moldovita nun Sister Tatiana. Peasants flocking to Sunday services from their mountain villages would have seen only high stone walls, built to defend against invading armies. The real treasures were hidden behind those walls: modest, three-room Gothic churches, covered from bottom to steeple-top with Byzantine iconography in vivid, intense colors. Voronet, built by Stephen the Great in 1488, is a deep blue; Sucevita, erected nearly a century later, is a grassy green; Moldovita's frescoes are filled with yellow accents, and Humor, built by Petru Rares in 1530 and covered in frescoes five years later at his direction, is primarily red.

Creating the frescoes took a sure, quick hand. Teams of four or five painters would first even out the church's rough stone walls with a thick layer of mortar, then smooth on a thin, fine-grained layer of lime plaster mixed with natural fibers such as finely chopped straw. Once the last layer was applied, the artists had only a few hours to work before the plaster dried. "They had to be very organized, like an assembly line today," says Oliviu Boldura, an art conservation professor at the National University of Arts in Bucharest.

Apprentice painters would apply background color and decorations, while faces and hands were reserved for master painters. Artists had to be chemists as well, mixing pigments from rare clays, semiprecious stones and common minerals. Azurite and malachite created vivid blues and greens. Ochre from clay was heated to produce reds, yellows and browns. As the plaster dried, it reacted with the mineral pigments, fixing the colors. The technique, which involved no organic materials, unlike frescoes that use egg whites as a binder, made the colors unusually durable. "Don't forget, even today exterior paints don't stay one year outside before changing color," Boldura says. "These have lasted for 450."

The Bukovina painters, most of whose names have been lost, also added local touches. In Voronet's magnificently preserved "Last Judgment," souls on their way to heaven wear locally embroidered cloth; angels announcing the last judgment blow shepherd's horns; and King David plays a Romanian cobza, or lute. "The painters weren't famous. They were folk painters," says artist and historian Eugen Mircea. "They were trying to make sure simple people could relate to the Bible stories in the pictures."

For sheer gore, some of the depictions of saints and martyrs rival Hollywood horror movies. Calm, beatific figures are shown being burned alive, dragged behind horses, thrown over castle walls, strangled, boiled and beheaded. Their tormentors are usually dressed as Turks.

Stephen the Great's victories against Turkey's Ottomans were, in fact, short-lived: Moldavia eventually became an Ottoman vassal in the mid-16th century, paying tribute in exchange for local autonomy and religious freedom. By the early 1600s, the principality was in decline—perhaps, Mircea suggests, because of too much spending on churches and frescoes. In 1775, the area became part of the Austrian Empire, and to suppress local impulses toward independence, the Austrians closed most of the monasteries. The rest fell into decline. The Communist era after World War II ended a century and a half of neglect. The brutal dictator Nicolae Ceausescu, who took power in 1965 and ruled until he was overthrown in a bloody revolution in 1989 (and, with his wife, executed), made the churches into national museums open to tourists, but not to religious orders.

By then, time had taken a toll; only a handful of the churches had significant frescoes left intact. Weather remains the main culprit. Mountain temperatures can range from 90 degrees Fahrenheit to -5 degrees in a single year, expanding and contracting the frescoes and creating tiny cracks that let in water. North-facing frescoes, exposed to the harshest winds and rain, are badly faded at best. Conservation work, which began in earnest 30 years ago, is ongoing.

Although Bukovina is one of the poorest corners of one of Europe's poorest countries, things are changing. Romania joined the European Union this past January, and Western-style resort hotels are springing up to serve the growing tourist trade. Since the fall of Communism, the churches have undergone a renaissance of sorts. Today most are thriving abbeys, home to large communities of Orthodox nuns who make a living farming and producing handicrafts. The nuns still find time to pray seven hours a day, drawing inspiration from the ancient visions on the walls. Adjusting her black shawl against the morning chill, Sister Tatiana regards her Moldovita sanctuary with special pride. "Each church has its own personality, its own history, its own meaning," she says. "But this church is my heart. There are so many symbols—the paintings have theology, philosophy, history. Every year I find something new."

Berlin-based Andrew Curry is a former editor at Smithsonian.