Simplify the following text for a Grade 8 native English-speaking person.
Original: Michelangelo died in 1564 , leaving the drum of the dome complete , and Bramante 's piers much bulkier than originally designed , each 18 m across .
Simplified: When Michelangelo died in 1564 , the walls were being built , the piers had been strengthened and everything was ready for the building of the dome .