Final answer:
The Colosseum features that were meant to be visible include the Arena, Facade, and Hypogeum. The feature that was not meant to be visible was the Velarium.
Step-by-step explanation:
The Colosseum was a massive structure in the middle of the city of Rome that was the site of many public entertainments, including gladiatorial matches. The features of the Colosseum that were meant to be visible include the Arena where the gladiator fights took place, the Facade which was adorned with monumental arches and statues, and the Hypogeum which was the underground area used for staging events and housing participants and animals. The feature that was not meant to be visible was the Velarium, which was a retractable awning system that covered the top of the Colosseum and provided shade for the spectators.