In the spring of 335 BC, Alexander III, the new king of Macedon, was climbing a hill in Thrace. It was not an ordinary hill, but a rocky stronghold of fortifications, houses and towers, protecting something famed across the ancient world: the Thracian sanctuary of Dionysus. There, the god's will was revealed when wine was poured onto the flames of a sacred altar – one that had never been known to fail in its predictions.

Young and ambitious, Alexander had come to learn his future. He did. As the flames rose higher than anyone had ever seen, the oracle spoke. Alexander was going to conquer the entire known world.

Soon afterwards, he crossed into Asia, crushed Persia, became a god in Egypt and fought his way to India, subjugating every people and kingdom in his path until he reached the limits of the known world.

For years, archaeologists have combed Bulgaria in their quest to find the site of the ancient Oracle of Dionysus. The strongest contender is Perperikon, deep in the Rhodope mountains, some 20 kilometres from Kardzhali.

Each season of archaeological digs brings new parts of the mighty city to light

Since active excavations started in the early 2000s, archaeologists have uncovered a remarkable place that over the course of millennia kept evolving alongside its inhabitants, their economy, society and religious beliefs. Perched atop a rocky hill, it covers over 1,200 acres – supposedly the largest megalithic site in the Balkans, a maze of altars, stairways, palaces, residential areas, chapels, churches, pagan temples and fortification walls all hewn out of, into or above the bedrock.

Perperikon began rather inconspicuously. Sometime in the 6th millennium BC, people started coming to the rocky hill to pray. Their rituals were hardly spectacular – they would carry some pottery up to the hill, break it and stick the shards into the rock's crevices as offerings to the unknown deities or spirits they venerated. Then, after centuries of repeating the same obscure ritual, the believers left Perperikon for good as inexplicably as they had appeared.

About 1,000 years passed without a trace of human presence there. At the end of the 5th and the beginning of the 4th millennia BC, however, the hill began to attract worshippers once again. By the 18th-12th centuries BC, ritual activity had changed: people started hewing niches, altars and basins into the rock. Gradually, they built a whole settlement, complete with a fortified acropolis, a mighty palace and residential quarters. The foundations of the buildings, the ground floors and even the streets and sewage systems were all cut into the bedrock.

The most impressive structure from this period is known as the Palace – a labyrinthine construction of more than 50 halls and rooms, corridors and covered staircases. It also contained a ritual hall with a large circular altar made of rock. There, a supposed throne was hewn of stone, while the surrounding rocks were covered with oval, rectangular and circular pits. Some were sacrificial, others served as storage spaces.

Churches, fortifications and necropoli created over the remains of earlier palaces, shrines and forts: Perperikon's ruins form a larger-than-life jigsaw puzzle in stone that only professional archaeologists and historians can decode, revealing what was built or carved, when and for what purpose

Perperikon went into decline in the 4th-1st centuries BC, but when the Romans took over Thrace, activity resumed and new buildings adorned the hill. The fortifications on the acropolis were reinforced.

The town's aura of sanctity was so strong that it survived even the arrival of Christianity, after the 4th century. The adherents to the new faith moved into the empty temples of the old one, and Perperikon became the stronghold of local bishops, remaining one for centuries. The walls of an early Christian basilica are now clearly visible on the site. Close to it are the ghostly remains of a monastic necropolis, with graves cut into the bedrock.

Perperikon was abandoned for good in 1362, when the Ottomans invaded. Nature soon swallowed up the remains of the churches and the Thracian shrines. The site was discovered by archaeologists in 1979-1982, but became a household name only in the early 2000s, when excavations restarted and the hypothesis that it was the Oracle of Dionysus was promoted as an established fact.

This medieval tower is the last standing structure at Perperikon

The hypothesis is supported by the sheer scale of evidence for religious activity spanning several millennia. According to ancient accounts, the shrine of the god of wine and divine madness was immensely popular and predicted the glorious future of both Alexander the Great and the Emperor Augustus. The Roman historian Suetonius records that when Augustus's father consulted the oracle in the grove of Bacchus about the future of his son and poured wine upon the altar, so prodigious a flame burst forth that it reached up to the heavens – something said to have happened only when Alexander the Great had sacrificed at the same altars. Ancient accounts, however, remain ambiguous on the oracle's exact location.

Archaeological research on the hill continues to this day and there is hardly a summer without some new discovery being announced. The site has been substantially developed in recent years – wooden walkways now criss-cross the excavations, guiding visitors through the layers of stone and history. Visitor numbers are counted in the thousands. For a place that spent eight centuries under a mantle of vegetation, Perperikon has proved remarkably good at making itself heard. 

When you stand on Perperikon and look around, marvelling at the surrounding peaks, villages and the river below, you understand the site's strategic position and, ultimately, its economic, religious and military significance through the millennia
 

Sometimes the city looks as if it were built by giants for giants