The Colosseum was originally not called that but instead the Flavian Amphitheatre (Latin: Amphitheatrum Flavium). People came to know that it stood opposite the Colossus of Nero, and, over time, the reference to 'Colossus' was transferred to the amphitheatre. The Colossus of Nero ceased to exist, but the Colosseum continued to be known by that name.
The guide told us it was opened in 80 AD, and had 80 entrances, through the arches that you see at the base of the structure (see below), and which could be found all the way around. Seventy-six were numbered (in Roman numerals), and the other four were not, as they were for important people like the Emperor. The people of Rome could come and enjoy many spectacles except in winter and summer. They didn't have to pay, but they did get tickets with seat numbers, and the people would enter the Colosseum and look for their seat, just like today. The spectacles involved lions, tigers and bears, and criminals would be punished by being placed in the arena with the animals. Also, gladiators would fight the animals. The Romans had incredible agency; not only could they build marvellous structures like the Colosseum, but they had the ability to go off to Africa and Asia and bring fearsome beasts back to Rome. If Rome had survived, humans might have gone to the moon a lot earlier than was the case.
Indeed, the Colosseum was a marvellous structure. It seated 60,000 people, according to our guide. It was elliptical; its two furthest points from side to side measured 600 feet. The arena floor was covered in sand. The Romans could manipulate the floor so that the sand would create dust clouds, during which wild animals would be raised through trap doors to the arena floor from the basement. When the dust settled, spectators would be surprised to find a lion and gladiator in front of them, with the action about to begin! Incidentally, the English word 'arena' comes from the Colosseum because 'rena' is Latin for 'sand'. The structure was so designed too that the arena could be flooded. Many naval battles took place on boats, all for the spectators' enjoyment. The arena no longer exists, allowing a good view into the basement where beasts, gladiators and criminals waited to be brought above.
The Colosseum had four layers. The first picture below shows the outer layer, which only exists in part. The seventh pic below shows where the outer layer comes to an end. The outer layer, and the Colosseum, began falling apart when centuries later the Popes started pilfering Colosseum materials for their own purposes. An earthquake in 1348 also did damage, and the Popes continued their pilfering. The Colosseum has undergone preservation work that stretches back to the mid-19th century. Some brick reinforcement dates to that time. We just had to come to the Colosseum, after all......when in Rome....