The Capilla Mayor faces the choir, merely separated from it by the space lying directly under the dome and forming the intersection of nave and transepts. As the church services constantly require the simultaneous use of the choir and the high altar of the Capilla Mayor, a portion of the intermediate space or "entre los dos Coros" is roped off during service time for the clergy to pass from one to the other. The Spanish taste for pomp and magnificence centres in all its extravagance about the high altar, while a more subdued richness characterizes the surrounding stone and iron work which encloses the sanctuary on all sides. Not only on the front, complementing and balancing admirably the facing reja of the choir, but on the western ends of the sides, immense ornamental iron screens bar the way. The front one is quite overpowering in size, rising some seventy-five feet above the altar. The Spaniard was equal to any undertaking in the days of early Hapsburg splendor under the pious Reyes Catolicos. With the aid of Sancho Munoz and Diego de Yorobo, a Dominican Friar, Francesco de Salamanca designed them (1518) and then superintended the welding, gilding and the final erection in 1523.
The east end of the Capilla Mayor is formed by the magnificent retablo, almost four thousand square feet in size. One is immediately struck by its immense proportions and the infinite amount of carving bestowed on it. Its great scheme was conceived in 1482 by the Flemish sculptor Dancart, evidently a man of prolific and versatile imagination. If we try to compare it with the work of English churches, we might best liken it to the great altar screens. This and the retablo at Toledo are probably the richest specimens of mediæval woodwork in existence. Portions of the execution are somewhat inferior to the conception, and yet the artists who labored on it with loving skill until the middle of the following century carried out all their work with a richness and delicacy which make it not only a representative piece of late Gothic sculpture but one of the most magnificent specimens of this branch of Spanish art. Its various portions embrace the whole period of florid Gothic from its earlier, more restrained expression to the very last stroke of the art, when wood was mastered and carved into incredible filigree work as if it had been as soft and pliable as silver leaf. Everything that could be carved is there, figures, foliage, tracery, moldings and mere conventionalized ornament. The central portions are of the earlier fifteenth century, the outer ones, of the late sixteenth, executed under Master Marco Jorge Fernandez. The wood is principally larch, with minor portions of chestnut and pine. The whole field is divided by slender shafts and laboriously carved bands into forty-four compartments representing in high and low relief various scenes from the life of Christ and the Virgin Mary. In the centre is Santa Maria de la Sede, the patron saint of the church, surmounted by a Crucifixion with Saint John and the Virgin on either side.