Simplify the following text for a Grade 6 native English-speaking person.
Original: The building was remodelled further under Vespasian 's younger son , the newly designated Emperor Domitian , who constructed the hypogeum , a series of underground tunnels used to house animals and slaves .
Simplified: 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 .