Simplify the following text for a Grade 7 native English-speaking person.
Original: Two settlements arose on both sides of the river : SlezskÃ Ostrava ( Silesian Ostrava ) was first mentioned in 1229 , MoravskÃ Ostrava ( Moravian Ostrava ) in 1267 , it received town privileges in 1279 .
Simplified: Ostrava is the third biggest city in the Czech Republic .