The imposing exterior walls of the sanctuary are almost entirely undecorated. There are three great, classically inspired doors with pedimented arches decorated with columns with Doric capitols. Unadorned white stone walls rise to the cornice that is also simply decorated with Corinthian columns. The outside of the building remains unfinished as work had to be stopped in 1566 when the builders discovered that the ground would not support the external dome or a campanile. It is the interior however, that bears such a striking resemblance to the interiors of both Santa Maria della Consolazione and Santa Maria at Loreto.
