The most extraordinary portion of the building is unquestionably its "fighting turret" and eastern end. This apse is almost unique in Spanish architectural history and deeply absorbing as an extensive piece of Romanesque work, not quite free from Moorish traces and already employing in its vaulting Gothic expedients. It may be called "barbaric Gothic" or "decadent Romanesque," but, whatever it is termed, it will be vitally interesting and fascinating to the student of architectural history. Externally the mighty stone tower indicates none of its interior disposition of chapels or vaulting. The black, weather-stained granite of its bare walls is alternately broken by slightly projecting pilasters and slender, columnar shafts. They are crowned by a corbel table and a high, embattled parapet, that yielded protection to the soldiers occupying the platform immediately behind, which communicated with the passage around the city walls. This is again backed by a second wall similarly crowned. The narrowest slits of windows from the centres of the radiating, apsidal chapels break the lower surfaces, while double flying buttresses meet, at the level of the triforium and above the clerestory windows, the thrusts of the upper walls. The plan is most curious, and on account of its irregularity as well as certain inconsistencies, it is difficult to guess how far it was originally conceived in its present form, or what alterations were made in the earlier centuries. Some changes must have been made in its vaulting. The chevet or Capilla Mayor, which at first very properly contained the choir, is surrounded by a double ambulatory, outside of which the thick walls are pierced by nine apsidal chapels. It is probable that these were originally constructed by the engineers to lighten the enormous bulk of the outer masonry. They are not quite semicircles in plan, and are vaulted in various simple ways. Where ribs occur, they meet in the key of the arch separating chapel from ambulatory. The piers round the apse itself are alternately monocylindrical and composite; the intermediate ones, subdividing unequally the "girola," are lofty, slender columns, while those of the exterior are polygonal in plan, with shafts against their faces. Some of the caps are of the best Romanesque types, and composed of animals, birds, and leaves, while others, possibly substituted for the original ones, have a plain bell with the ornamentation crudely applied in color. The Capilla Mayor has both triforium and clerestory of exquisite early work. Dog-tooth moldings ornament the archivolts. Mohammedan influence had asserted itself in the triforium, which is divided by slender shafts into two windows terminating in horseshoe arches, while the clerestory consists of broad, round, arched openings.