Palazzo di Re Enzo. This Gothic palace was once the prison of the son of Frederick II, Enzo. It was not originally Gothic; those design elements were added in 1905 when the palace was restored. According to legend, Enzo could roam the palace during the day but was locked in a cage that hung from the ceiling at night.
Palazzo Comunale. Also known as Palazzo d'Accursio, this is located at Piazza Maggiore and serves as the town hall. It holds the Civic Art collection and the Museo Morandi. The facade has beautiful portcullis and terracotta decoration.
The Archiginnasio. Originally part of the University of Bologna, now a library. Commissioned by Pope Pius IV, construction finished in 1563. The first floor has an anatomical theatre with statues of famous doctors around the walls -- important doctors got full-length statues, lesser ones only busts.
Fountain of Neptune. Right next to the Piazza Maggiore, depicting Neptune on top of four angels and four sirens. The angels represent the four winds, the sirens the four known continents when the fountain was built. Designed by Tommaso Laureti, with a notable innovation: the water basins do not follow geometric forms.
Santo Stefano. A complex of churches, crypts, and chapels that interconnect. Originally there were seven churches, now four remain: Chiesa del Santo Sepolcro, Chiesa del Crocifisso, Chiesa del Calvario, and Basilica dei Santi Vitale. Most are surrounded by courtyards. This was a pagan worship site before the birth of Christ. The churches were built between the fifth and twelfth centuries.