It is a brick edifice with a portico on the ground floor and a large hall on the upper.
The present fabric is largely modern; while the portico was designed by Inigo Jones.
The main facade of the cathedral was largely altered in 1491, to which date the statues upon it belong; the portico in front was added in 1497.
At the other end of the great hall is a similar portico facing outwards; and between this and the doors the hall is divided into three aisles by rows of Ionic columns.
Among the first of these benefactions was the great gymnasium of Ptolemy, built in the neighbourhood of the Agora about 250 B.C. Successive princes of the dynasty of Pergamum interested themselves in the adorn western entrance being the well-known Doric portico of Athena Archegetis with an inscription recording its erection from donations of Julius Caesar and Augustus.