Simplify the following text for a Grade 6 native English-speaking person.
Original: The nave is generally considerably taller than the aisles , having clerestory windows which light the central space .
Simplified: The nave is usually a lot taller than the aisles , and has high windows which light up the central space . The upper part of the building , where these windows are , is called the clerestory ( or clear storey ) .