<human>:  When did the Romans start building the Colosseum?
<bot>: The Colosseum was built by the Roman Emperor Vespasian in 72 AD. The Romans began building the Colosseum in 72 AD because they were planning to build a huge amphitheater to host the gladiator games. The gladiator games were a popular event in ancient Rome, and they were held in the Colosseum. The gladiator games were popular because they were a good way to train soldiers and test their strength. The gladiator games were held in the Colosseum until about 200 AD.