Second in size only to the Colosseum in Rome, this Roman amphitheater was probably built around A.D. 3, enlarged in 119 by Emperor Hadrian, and further embellished by Emperor Antoninus Pius. It remains majestic in spite of having been used -- like so many Roman buildings in Italy -- as a quarry for quality marble and construction materials over the centuries. It was also picked apart in the 9th century onwards during searches for bronze and lead (the building's large stone components were broken apart to get at the heavy metal clamps that held them together). Some of its columns and stones were even used to rebuild the town and the Duomo.
Judging from what remains, though, the amphitheater must have been quite a sight back in those days. Four stories tall, it was completely covered in travertine stone, with marble busts of gods serving as keystones for each of the 240 arches on the lower floors, and full-length statues under the arches of the second and third floors. You can still see the carved keystones over the main entrance (busts of Ceres and Juno). The giant arena has a maximum length of about 170m (558 ft.) and can seat over 60,000 people. The corridors below the arena are relatively well conserved and still show traces of stuccoes and frescoes. This is where the gladiators waited between combats, and where all the scene props were kept. The bestiary -- home of the fighting animals -- was also here. You can see the remains of a paleochristian altar and paintings inside one of the small rooms here, which was transformed into a Christian oratory in the 9th century.
The garden in front of the amphitheater has been turned into an open-air museum where you can admire many of the fragments of the original decorations of the amphitheater as well as from other buildings in town. Among the objects on display is a beautiful 2nd-century mosaic depicting Nereides and Tritons.
A permanent exhibit dedicated to gladiatorial fights is housed in the Museo dei Gladiatori (Gladiators' Museum), also in the garden. On display are four complete suits of gladiator armor, and a model reconstruction of the amphitheater. You can also watch a good animation of a gladiator fight. Spartacus, the slave made famous by the 1960 Stanley Kubrick film, was a graduate of the gladiator school located near this amphitheater.