Hope this helps, lifted from a site about Castles and such like:
Traitors Gate
The History of the Tower of London is bloody and cruel and Traitor's Gate has its part to play in its story. The water-gate under St Thomas's Tower has been known for over 400 years as 'Traitors' Gate' because of the number of prisoners, accused of treason, who have passed through it. Unfortunate and important state prisoners were committed to the Tower of London through the River Thames entrance to the Tower of London called Traitors Gate. The journey of these prisoners was made by barge along the River Thames. Often their journey would take them past London Bridge where the heads of recently executed traitors were displayed on the roof of the stone gate house. The heads were placed on spikes, attached to poles and displayed on the Bridge. The young and tragic Catherine Howard had to pass the gruesome sight of the head of her lover Thomas Culpepper on her journey to the Tower of London and Traitors Gate. This grisly practice continued until around 1678.
Traitors Gate - the famous entrance to the Tower of London
The famous entrance to the Tower of London, Traitors Gate, can be clearly seen by anyone passing up or down the River Thames. This infamous water-gate entrance to the Tower of London was designed by the Medieval architect Master James of St George on the orders of King Edward I between 1275 and 1279 as part of St Thomas's Tower. It was originally built to provide a new water-gate by which King Edward could arrive at the Tower by river. St Thomas's Tower provided additional royal accommodation for the King and his family. The function of the Tower of London was originally to provide royal power base in the City of London and to provide a retreat for the Royal family in times of civil disorder. This changed over the years and was increasingly used as a prison for enemies of the state accused of treason which was entered via Traitors Gate. The Tower was last used as a Royal residence by Anne Boleyn when she was preparing for her coronation. It was a tradition that new Kings and Queens of England would make their way from the Tower of London to Westminster Abbey for their coronation. The death of Anne Boleyn heralded the increased use of the Tower as a prison. The water-gate under St Thomas's Tower has been known for over 300 years as 'Traitors Gate' because of the number of prisoners, accused of treason, who have passed through it.
It looks like just the outer wall is now bricked up, so no access direct from the Thames anymore.