Simplify the following text for a Grade 6 native English-speaking person.
Original: However , when he arrives , Mario discovers that Bowser has invaded the castle and imprisoned the princess and her servants within it using the power of the castle 's 120 Power Stars .
Simplified: By the time Mario arrives , however , Bowser has taken control of the castle and has kidnapped Peach and stolen the power of the castle . Mario must save the princess and the kingdom before it 's too late .