Simplify the following text for a Grade 11 native English-speaking person.
Original: The southern gatehouse , the Stone Gateway , became the scene of one of London 's most notorious sights : a display of the severed heads of traitors , impaled on pikes and dipped in tar to preserve them against the elements .
Simplified: The southern gatehouse , the Stone Gateway , became the scene of one of London 's most notorious sights : a display of the severed heads of traitors , stuck on pikes and dipped in tar to preserve them .