The Colosseum had been completed up to the third story by the time of Vespasian's death in 79. The top level was finished and the building opened by his son, Titus, in 80. Cassius Dio said that over 9,000 wild animals were killed during the opening games. The building was changed by Vespasian's younger son, Emperor Domitian. He added the "hypogeum", underground tunnels used to hold the animals and slaves used in the games. He also added a fourth level at the top of the Colosseum to add more seats.