Each temple in ancient Greece was dedicated to only one god. Because the Greeks worshiped many gods, there were a great many temples in ancient Greece.
The most famous temple dedicated to Apollo was the temple at Delphi. That temple was the home of Apollo's special oracle, a young woman Apollo had gifted with the ability to see the future. Apollo could do things like that. Apollo had many magical powers, and many jobs to do in the ancient Greek world. But he always found time now and then to visit the many temples built in his honor.
One day, he heard about an especially splendid temple that had been built in the city-state of Troy, a temple like many others built in his honor. The interesting thing about this temple was that the city-state of Troy was not located on the Greek peninsula. Troy started as an ancient Greek colony. Troy was on a different peninsula entirely.
Being a curious god, one day Apollo decided to swing by and take a look at the temple at Troy for himself.
As the story goes ...
Cassandra was not an oracle. She could not see into the future. She was a beautiful young priestess, with great ambition.
When Apollo swung by personally to take a look at his temple, all Cassandra saw was his beauty and his power. She gave him a saucy grin.
Apollo was delighted. Like all the gods, Apollo was very fond of making deals. He offered a deal to Cassandra. If she would give a kiss, he would give her the gift of prophecy so she could see into the future.
Cassandra did not hesitate. She took the deal. As soon as Apollo gave her the gift of prophecy, she looked eagerly into the future. What she saw made her gasp. She saw Apollo helping to destroy her beloved city of Troy. She spit in his face.
Apollo was angry of course. But could not take away his gift. He could only add to it. That's how his power worked. That's exactly what he did. He added something to his gift. From that time on, Cassandra could see the future, but no one believed a thing she said. That's what Apollo had added.
Some time later, Cassandra warned her people that the huge wooden Trojan horse the Greeks had given Troy was hollow - full of Greek warriors, hidden inside - warriors who would destroy the city! The people of Troy did not believe her. They did not even look. They were far too busy celebrating the "defeat" of the Greeks, who had left the horse as a token of defeat (or so they believed.) But the horse was full of Greek warriors. That night, they crept out of the hollow horse and opened the city gates to let the Greek army in. That was the end of Troy.