St Thomas's Tower was built by Edward I between 1275 and 1279 to provide accomodation for the King and a new water entrance to the Tpwer, replacing the watergate of the Bloody Tower behind it. This river entrance is now often referred to as Traitor's gate because of the number of prisoners accused of treason that are supposed to have passed through it. For prisoners such as Queen Anne Boleyn and Sir Thomas More the trip was to be their last.
In the pool behind traitors' gate was an engine which was used for the raising of water to a cistern on the roof of the White Tower. The engine worked originally by the force of the tideor by horse power and eventually by steam. In 1724-26 it was adapted to drive machinery for boring gun barrels (it was removed in the 1860's).