Very severe and imposing building entirely made of brick, in the monastic Gothic style, with a rectangular nave and beamed roof.

Outside :
From the space in front of the Abbey a most peculiar and picturesque view of Siena is to be admired; it is a multi-colored picture with different hues of red, yellow and grey. High up on the left is the bright stumpy Ballati tower, in the middle the red Mangia tower, on the right the black and white striped structure of the Cathedral. In the cloister (1425) on the right side of the façade, are fragments of some beautiful frescoes by Lippo Memmi and Andrea Vanni

Inside: this Gothic Church has the shape of Egyptian cross with single nave. Three Chapels are on the each side of the high altar.
At the bottom of each nave is the "Cappella delle Volte", where Catherine Benincasa (St. Catherine of Siena) used to linger in prayer and ecstasy (an inscription on the pillar reminds the visitor to this). At the bottom of the Chapel is the altar with the famous fresco of the Saint, by Andrea Vanni, which is believed to be the Saint's only portrait. Then from the left to the right are several pictures with scenes from Catherine's life.

Over the altar are more works of art:
- San Giacinto by Francesco Vanni
- St. Anthony by Rutilio Manetti
- The Virgin and the Holy babe with St. Jean the Baptist is a detached fresco ascribed to Pietro Lorenzetti
- Madonna by Stefano Volpi.
- The Nativity of Virgin Mary by A. Casolani.
- In the lunette is Matteo di Giovanni's "Pietà"
- On the altar foot-board is "Stories of Saints"


On the right side of the nave is the "Cappella di Santa Caterina" ( St. Catherine of Siena Chapel), built in 1488. Within a recently made reliquary (1940) shaped like a Gothic temple, is a treasured the Saints head.
This chapel is almost entirely frescoed by Sodoma and on the altar is the painter's masterpiece: "St: Catherine Swoon", famous for the figure of the Saint which is almost seems to be breathing; the admirable harmony of the whole group and the pretty landscape are delightful.
The staircase beyond Cappella delle Volte leads down to the Crypt where, on the high altar is Sano's di Pietro's famous "Crucifix". The wooden statue on the right is Vecchietta's "St. Anthony".

The High altar has a very fine marble ciborium made by Giuliano da Maiano.

Other important works of art :

Madonna and Child by Sano di Pietro.
Epiphany by Matteo di Giovanni.
Madonna and Child by Benvenuto di Giovanni.
St. Bernardine of Siena ascribed to Sodoma.