The subsequent expansion was managed, between 1317 and 1326, by the architect Ambrogio Maitani from Siena who, with his brother Lorenzo, was hired to refurbish the Montepacciano aqueduct, which carried the water to the fountain in the square.

During this same period, lodgings were built for the new Magistracy of Priori (founded in 1303, consisting of ten representatives of each one of the main guilds of the arts, among the 44 that existed; they held office for only two months, in which they were housed in the palace).

Two large corbels above the Gothic portal support copies of bronze statues of the griffin, symbol of the city, and the Guelph lion; the originals (now in the entrance hall of the palace) were made in 1274,[4] probably for a vanished fountain by Arnolfo di Cambio.

On top of the pillars are two griffins, overlooking calves, the symbol of Corporazione dei Macellai (the guild of butchers), which commissioned the work.

[5] The style and the size of the portal are in contrast with those of the palace; for this reason, it is thought to have been constructed for an ecclesiastical building, but was later used for Palazzo dei Priori.

On the left side of the portal is a figure with a title that reads: Entra Puro, move securo ('Come thither, be safe').

In 1860 the frescoes were heavily restored by Perugian artist Matteo Tassi, who added neo-Gothic and eclectic decorations throughout, and repainted the emblems of successive podestà and capitani del popolo in the city's government.

At the top, on the right wall was a passage connecting to the residence of the capitano del popolo.Located on the north side of the building, it is accessible through the stairs on the right of the portal to the Sala dei Notari or from inside on the main floor.

The small entrance door has shutters covered by an iron plate with carved griffins and in the centre the letters A.G., meaning Armadium Generale, i.e. the first municipal archives.

On the back wall is a fresco from a small country church depicting the Nativity and saints, made by Tiberio d'Assisi (15th-century Umbrian school).

In 2010 a work by Perugian Mannerist Antonio Maria Abbatini (1598–1680), characterized by a large number of figures in a very small space, was placed there.

On the first floor is the Comune council chamber (Sala del Consiglio comunale), where above the door is a lunette with a fresco by Pinturicchio, depicting the Virgin Mary between two angels.

Also preserved is the original Pietra della giustizia ('stone of justice'), dated 1234, which was publicly displayed in the main square (Piazza IV Novembre).

[7] Ceramist Giacomo di Marino, known as il cavalla, made the glazed majolica floor between 1455 and 1457, decorated with Gothic-floral motifs on a blue background, alternating with flying angels anticipating a Renaissance style.

Italy, which contains the three perfect ceilings of the world – those of Mantegna in the ducal palace of Mantua (see Plate XXV), of Perugino in the Sala del Cambio at Perugia and of Araldi in the Convent of St. Paul at Parma – is the best field for the study of this branch of art.

"[8] The Collegio della mercanzia, located in the second fondaco to the right of the Portale Maggiore, is entirely covered in late Gothic wooden decoration.