Megalopolis, as the name suggests, was a major regional centre in Arcadia, the first one of the kind when it was founded around 370BC as a counterweight to Sparta. Nowadays it remains the largest town of the area (though the capital of the whole of Arcadia is a much-larger Tripoli) and sits in this valley, flanked somewhat unromantically by two power plants (which apparently supply great proportion of electricity needed in the whole of Peloponnese). Despite sounding very grand, Megalopolis is a sleepy little place with a population barely above five thousand people, a small market town with a few shops and bars, but busy with locals that come from the surrounding villages.
For a tourist the chief interest in Megalopolis is the ancient site, located somewhat out of the current town. The site was excavated in the late 19th century by the British Archaeological School at Athens, while new work by Greek archaeologists started in the 1990s and is still going on.
The main structures in ancient Megalopolis include the ancient theatre, agora, parts of walls and the Thersileion Bouleuterion. Of these, the agora with remains of an impressive stoa (covered portico, now open to the skies but with many columns standing) is clearly visible from the road north towards Karitena and Gortyna and can be easily explored on foot.
Megalopolis' theatre used to be the largest in the ancient world, with a capacity of 20,000 people. Its remains can be seen closer to the modern town, though you need to turn-off the main road to access the site (there is parking available). At the time of our visit it was officially closed (and it's fenced off, though you can see bits through the fence) but there was a group of volunteers working there, helping the excavations and clearing some of the ground and stones, and thus we were luckily let in, even with a little background information.
What is left now gives only a some impression of the original scale of the place as most of the tiers of seats that surrounded the stage are still covered by a bank of earth, but several tiers at the bottom have been cleared and the structure is emerging out of the earth closer to its former glory.
Certainly worth a peak if you are driving past or staying locally.