Simplify the following text for a Grade 15 native English-speaking person.
Original: The kingdom of Castile ( Castilla ) built a global empire , spreading its culture and language in the Americas and biologically and culturally mixing with the Indigenous peoples and ultimately , the African slaves , altering the composition of the hemisphere .
Simplified: The kingdom of Castile ( Castilla ) built a global empire , spreading their culture and language in the Americas and biologically and culturally mixing with the indigenous peoples , altering the composition of the hemisphere .