Simplify the following text for a Grade 4 native English-speaking person.
Original: Silla was founded by King Park Hyeokgeose in 57 BC , around Gyeongju in the southeast part of the Korea peninsula . Although Silla was founded earlier than Goguryeo and Baekje which coexisted with Silla during that time , it took more time for Silla to build firm structure form of country than those .
Simplified: The capital of the Silla kingdom was Gyeongju .