The most famous ancient sanctuary in Western Europe was in Cadiz, in old Spain.
Hercules was buried there.
Hercules Monument at Cadiz, Spain
There was an oracle. You could ask her questions and get a cryptic reply or an interpretation of a dream.
The Oracle of Delphi by John Collier
Some of the most famous men of ancient history came as pilgrims to ask her for advice.
Hannibal stopped in to consult the oracle before he set off across the Alps with his army.
Julius Caesar asked the oracle to interpret a dream he’d had that troubled him. (He raped his mother. “Not to worry,” said the oracle in perfect Greek. “That wasn’t your mother but the world. The dream means you will conquer the world.”)
Pliny was there, Polybius, Cassius Dio, many of the Roman emperors, like Trajan. The Emperor Caracalla had the proconsul Aemilianus murdered for asking the Cadiz oracle who the next emperor would be.
For nearly one thousand five hundred years the sanctuary did service. As late as 400 AD it was still open when the poet Aviennus went for a visit.
What was it like?
It looked from the outside like a Greek temple. Ancient coins show it. The facade was a triangular pediment or gable supported by four columns. You walked up the steep steps, passed through the two central columns and entered through enormous bronze doors. On the doors were reliefs showing the Twelve Labors of Hercules.
Hercules. Gilded bronze, Roman artwork, 2nd century BC (H. 2.41 m (7 ft. 10 ¾ in.); in the Capitoline Museum, Rome (Creative Commons Attribution-Share Alike 3.0 Unported license photo by User:Tetraktys)
Somewhere near the entrance were two columns—the two “Pillars of Hercules”. The ancient authors don’t agree on what they looked like.
The Pillars of Hercules Monument at Jews’ Gate, Gibraltar (public domain photo by Julo (talk)
According to Posidonius they were eight cubits tall (ten feet?) and made of bronze; but Philostatus says they were of gold and silver and only one cubit high. An ancient Phoenician inscription on them had already become illegible by Roman times.
The temple was only partially roofed, so inside it wasn’t dark like Roman temples. The altar at the far end was open to the sky. Every day celibate priests with shaven heads and wearing snow-white tunics sacrificed a lamb or a dove and sprinkled its blood on the altar.
A perpetual fire burned on a tripod but there was no statue of the god. The Carthaginians, who founded the temple towards the end of the second millenium BC, didn’t allow images of their divinities. They had dedicated the shrine to their god Melkart, whose famous temple they had left behind in Tyre, at the other end of the Mediterranean. The Greeks came and Melkart became Herakles, a Greek god who was so similar to the Phoenician one that everyone just let one do for the other. To the Romans Herakles was Hercules—they were all names for the same god-hero. He was buried in a crypt under the temple—the mortal half of him.
There were various chapels or side-altars. One was dedicated to Old Age; others to Poverty, to Art, to Death, to the Month, and to the Year. Perhaps they looked like old attics, filled with strange wax exvotos and dried cloth, wooden and metal undefinables (weapons?), covered with the dust of ages.
There were relics, too, such as Teukros’ belt and Pygmalion’s miraculous olive branch (the olives were emeralds).
Its Great Treasure
The Temple received donations, legacies, and votive offerings from all over. Its treasury must have looked like Uncle Scrooge’s money-silo—gold all the way up to the top.
Uncle Scrooge by Walt Disney Productions (a free-use Wikipedia photo)
Hannibal’s relative Magón was the first to loot it in 206 BC.
Bogud, the king of Mauritania, tried to loot it in 38 BC.
The unscrupulous Roman consul Varron did loot it; but Caesar had the treasure restored.
My source for most of these facts is Historia de la Hispania Romana by A. Tovar and J.M. Blazquez, Alianza Editorial, 1975