Basilica of Sant'Andrea, Mantua

The facade of Sant'Andrea presents one of the most harmonious and intellectually coherent compositions in Renaissance architecture. The entire composition fits within a perfect square (width equals height), emphasizing geometric clarity and mathematical proportion—fundamental Renaissance ideals. The facade is divided into two distinct registers, yet unified through carefully coordinated proportions and the integration of classical orders. The lower register is dominated by an enormous central arch rendered in the manner of a Roman triumphal arch. This arch is extraordinarily tall and projects deep into the facade, creating a recessed barrel vault that serves as the entrance porch. The barrel vault is decorated with terra-cotta rosettes arranged in geometric patterns—an economical yet effective ornamental choice that demonstrates Alberti's pragmatic approach to architectural beauty. The central arch is flanked by two shorter fluted Corinthian pilasters, which stand at a smaller scale than the arch, creating visual hierarchy. On either side of the central arch are two smaller square-topped openings, much more modest in scale and proportion, yet proportionally coordinated with the central element. This arrangement—one large central opening with smaller flanking openings—directly echoes the organization of ancient Roman triumphal arches such as the Arch of Constantine and the Arch of Trajan at Ancona. The upper register employs a different classical order. Here, four giant unfluted Corinthian pilasters, significantly taller than the lower fluted pilasters, rise from the entablature to carry an entablature and triangular pediment. These pilasters span the entire width of the upper facade, creating an appearance of a classical temple front. This integration of a lower order of fluted Corinthian columns with a giant order of taller unfluted pilasters is described as a "novel aspect" of Renaissance architecture.

The interior is purposefully dimly lit. After the pilgrim's eyes adjust to the reduced light, the space becomes spiritually transformative. The uninterrupted barrel vault draws the eye along the length of the nave toward the high altar and the sacred relic housed in the crypt below. The unbroken cornice running from entrance to altar creates an unobstructed sight line, focusing the viewer's attention inexorably toward the sacred center. This orchestration of light, space, and visual focus demonstrates Alberti's command of what one scholar calls "Renaissance one-point perspective" applied to architectural space. Just as a painter uses perspective to fix the viewer's gaze on a focal point in a painting, Alberti uses architecture to fix the pilgrim's gaze on the sacred relic at the altar—the spiritual center of the entire composition. The deep porch creates a psychological and spiritual transition: the pilgrim exits the bright exterior piazza, ascends the steps, passes through the massive triumphal arch, and enters the dimly lit, vaulted interior space. The porch is a preparatory space—neither fully exterior nor fully interior—that prepares the pilgrim psychologically for entry into the sacred interior.

The interior of Sant'Andrea represents a radical departure from medieval church planning. Rather than the traditional basilica plan with a long nave flanked by side aisles lined with columns, Alberti designed a single, unobstructed nave. The nave is vast: 60 feet wide, 90 feet tall, and almost 300 feet long. It is roofed by a continuous barrel vault spanning the full width—18 metres (approximately 59 feet), making it the largest barrel vault constructed since classical antiquity. Instead of side aisles, the nave is flanked on both sides by three barrel-vaulted chapels. These chapels branch outward from the main nave at right angles, creating a series of opening niches in the nave wall. The chapels vary in size: larger open chapels (called Tribunalia) alternate with smaller closed chapels (cella). This arrangement provides additional space, allows pilgrims to view relics or pray in subsidiary locations, yet maintains visual continuity with the primary nave space. The vault is supported by huge piers rather than columns. These piers face the nave with pairs of giant pilasters that visually correspond to the exterior proportions—demonstrating Alberti's sophisticated coordination between internal and external elevations. The outward thrust of the barrel vault is counteracted by the structural buttressing provided by the side chapels. The vault was originally planned with coffered decoration, but lack of funds forced the coffers to be painted rather than carved.