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.