Near the foot of Kadifekale Hill, just off 920 Sokak (920 Street), this was the Roman city's market. The present site is a large, dusty open space surrounded by ancient columns and brick foundations, with decent signage in English. Alas, a substantial part has been closed off for excavations, though there are still some ruins to satisfy classical history buffs, and an interesting collection of Ottoman gravestones heaped near the entrance. (You can also walk down the hill to the Agora from Kadifekale, though it's not a very picturesque stroll.) If you want gobsmacking ancient grandeur, head for Ephesus, but if you're in İzmir, the Agora makes for an interesting excursion. Ask for a brochure at the entrance.
Off Eṣrefpaṣa Cad., 920 Sok., İzmir, İzmir, 35000, Turkey