Rome was founded in 753 BC by the twin brothers Romulus and Remus and has been recognised as a UNESCO world heritage site due to its many historic monuments. The Colosseum, built in the 1st century AD, was capable of holding over 50,000 spectators for gladiator battles and animal fights. The Mausoleum of Hadrian was originally built in the 2nd century AD as an emperor's tomb but later served as a refuge for popes. The Sistine Chapel, located in Vatican City, was commissioned by Pope Sixtus IV in the 15th century and is where papal conclaves select new popes.