Simplify the following text for a Grade 5 native English-speaking person.
Original: In 217 , the Colosseum was badly damaged by a major fire ( caused by lightning , according to Dio Cassius ) which destroyed the wooden upper levels of the amphitheatre 's interior .
Simplified: In 217 , the Colosseum was badly damaged by fire . Dio Cassius said the fire was started by lightning .