Simplify the following text for a Grade 7 native English-speaking person.
Original: Castella is a popular Japanese sponge cake made of sugar , flour , eggs , and starch syrup , very common at festivals and as a street food .
Simplified: Castella ( Kasutera in Japanese ) is a sponge cake produced in Japan . It is made of sugar , flour , eggs , and starch syrup , very common at festivals and as a street food .