San Sebastiano, Mantua

The plan is in the shape of a Greek cross, with three identical arms centering apses, under a central cross-vaulted space without any interior partitions.

The façade concealing a narthex that runs the full width of the structure is precisely as wide as its height from the entrance level to the apex of the pediment; it may be fitted with the perfect geometry of the square.

The temple front has been converted by Alberti into wall-architecture, as Wittkower noted, and a complete series of pilasters, like pillars embedded in the wall, has been elided to the two outermost, and the two flanking Pellegrino Ardizoni's clumsy doorway,[3] which overlaps them and ill suits its space.

The two outer staircases were added in the twentieth century; prior to 1925 old photographs show the entrance was a single stair to the quattrocento loggia appended to Alberti's design.

Wittkower demonstrates that Alberti's plan comprised a set of stairs the full width of the façade leading to five doors (three of which have been filled in as dysfunctional balconies).