This is the "A Saint with a Fortress and a Banner" by Girolamo da Santacroce. Painted in the Italian north, around 1512 to 1525, it's an oil-on-wood panel that measures approximately two feet high and one foot wide.
The saint, who may be Terentius, patron of Pesaro, stands on a parapet against a vast expanse of sky. The fabric of his cloak catches the light, revealing the intricate pattern of gold damask weave in tiny hatched brushstrokes. The way the light interacts with the brocade robe is also noteworthy.
In the background, a shepherd plays his flute beneath a tree while his sheep graze nearby. This composition would have been part of a larger altarpiece, known as a polyptych, which was a traditional form in this region during that time period.
The artist's style here is reminiscent of Giovanni Bellini's late works, and it's likely that Girolamo da Santacroce trained with Gentile Bellini in Venice. The treatment of the drapery and the oval shape of the saint's face are all characteristic of Bellini's style.
A Saint with a Fortress and a Banner hangs in The National Gallery on Trafalgar Square, London — free to enter. Point your phone at any artwork there and audioguide.london plays a free audio guide in six languages — no app download needed.