In 1900 by order of Cardinal Sancha for more security and custody, the room of the Treasure was fitted out under the tower. Here there are some of the main gold and silversmith works of art of the Cathedral, which are kept inside glass cabinets. Above them hang the paintings for the cardboards of the tapestries that were to be made later at mills of Saint Barbara in 1750. The most remarkable piece of these treasures is the great Processional Monstrance, commissioned by Cardinal Cisneros to Enrique de Arfe in 1515, that holds the Ostensory (inner small monstrance) in its centre. This was acquired by Cardinal Cisneros from the legacy of Queen Isabella the Catholic, and it is said that it was made with the first gold to arrive from America.
Enrique de Arfe constructed the monstrance following the design of the towers of Gothic cathedrals and the monstrance of the main altarpiece. It measures 2.50 m high and it is divided into three sections which are made up of 5,600 pieces joined by 12,500 screws. It also contains 250 enamel and golden silver small statues. It is crowned by a diamond Cross made in 1600 and it rests on a silver pedestal from the eighteenth century donated by Cardinal-infante Luis Antonio de Borbón.
Besides many other art works in the Treasure Room lies the exquisite Bible of Saint Luis from the royal French workshop that was given to Alfonso X in 1258. Other important gold and silver works can be admired inside their glass confines.