Short answers, pulled from the story.
Cybele is an Anatolian mother goddess originating in ancient Phrygia, where she was probably the highest deity of the Phrygian state. Greek colonists adopted her cult around the 6th century BC, and Rome made her a state deity in 204 BC under the name Magna Mater, meaning Great Mother. She was associated with mountains, wild animals, the protection of cities, and the cycle of life and death.
Rome officially adopted Cybele's cult in 204 BC, during the 14th year of the Second Punic War. A Sibylline oracle had declared that a foreign enemy threatening Italy could be repelled if Rome brought the goddess from Pessinos in Phrygia. A black lump of meteoric iron, venerated at Pessinos as the goddess herself, was transported by ship to Rome's port at Ostia.
The Megalesia was Cybele's major Roman festival, beginning each year on the 4th of April, the anniversary of her arrival in Rome. It included theatrical performances called ludi scaenici, probably staged on the stepped approach to the Palatine temple. On the 10th of April, her image was taken in public procession to the Circus Maximus, where chariot races were held in her honor.
The Galli were Cybele's eunuch mendicant priesthood, forbidden from Roman citizenship and rights of inheritance. The highest-ranking Gallus at Pessinos held the title Attis, and his immediate subordinate was called Battakes. The emperor Claudius later introduced the senior office of Archigallus, who held full Roman citizenship and was not required to be a eunuch.
The Taurobolium was a ritual bull sacrifice available from the 160s AD as a lawful substitute for self-castration for those seeking initiation into Magna Mater's mysteries. The Criobolium was a lesser version using a ram. Notable initiates included the praetorian prefect Praetextatus and the twice-consul Volusianus; both rites drew their participants from Rome's highest social class.
The Fountain of Cybele stands at the center of the Plaza de Cibeles, one of the endpoints of the Paseo del Prado in Madrid. Fans of Real Madrid and the Spanish national football team celebrate their victories around it, making the ancient Anatolian goddess a symbol of the city and the club.