Simplify the following text for a Grade 7 native English-speaking person.
Original: Later in 1638 , the grandson of Tokugawa Ieyasu , Matsudaira Naomasa enters the castle since the Horio clan had no heir , and his family ruled until the abolition of the han system .
Simplified: Later in 1638 , the grandson of Tokugawa Ieyasu , Matsudaira Naomasa entered the castle because the Horio clan had no heir . Matsudaira 's family ruled until the end of the han system .