The Tau? Yes, "T", in Greek, which is the form this palace had in the Middle-Ages... Raised between 1498 and 1509, rebuilt between 1686 and 1693 by the famous architect Robert de Cotte in the classical style (the current building). This is actually the former palace of the bishops!
In the past, we found a small palace, mentioned in 1138 as palatio Tau when bishops attended to king Louis VII's coronation. It was sold during the French Revolution: king Charles X and his family lived here for a moment and refit apartments (a study, some living-rooms, the king's room, kitchens on the cellar and a huge vaulted room).
Nowadays property of the State, the palace houses the cathedral's treasure! You can visit it, likewise the 13th century chapel (the oldest part of the palace) and the huge room flanked by a big chimney, fit by the archbishop Guillaume Briçonnet in 1498.