What major alteration was made to the Colosseum as it was constructed?

The addition of the hypogeum is significant because this underground structure was an essential innovation that transformed the functionality of the Colosseum. The hypogeum consisted of a network of tunnels and chambers situated below the arena floor, which were used for a variety of purposes, including housing animals, gladiators, and stage machinery. This design allowed for elaborate and dramatic entrances of combatants and animals, enhancing the overall experience of the games held in the Colosseum.

The hypogeum also facilitated the logistical needs of the spectacles, as it provided space for the storage of props and equipment needed for the elaborate performances that characterized Roman entertainment. This element also reveals the advanced engineering and architectural capabilities of the Romans, as they were able to create a multi-layered structure that supported both the arena above and the complex operations below.

In the context of the other choices, while various modifications and adaptations were made to the Colosseum during its construction and use, the addition of the hypogeum stands out as a crucial feature that directly affected the nature of events held at this iconic site.