This Oracle, dedicated to Amon, was visited by Alexander the Great in 332 BC, when he was seeking confirmation that he was the son of Zeus. Nobody knows what the oracle told Alexander, the answer was whispered into his ear. But it probably was confirmative; Alexander expressed ever since a wish to be buried at Siwa, and he embarked upon great conquests in the east, conquests that only a son of a god would dare to embark upon.
The large tower is not authentic, but build later. It is a mosque.