This was the city’s market area under Roman rule, and it occupied a much larger area than the current site borders. You can see a lot from outside the fence, but it's worth going in for a closer look at the well-preserved Gate of Athena Archegetis, the propylaeum (entrance gate) to the market, as well as an Ottoman mosque and the ingenious and beautiful Tower of the Winds, on the east side of the site.
The gate, formed by four Doric columns, was financed by Julius Caesar and erected sometime during the 1st century AD. To the right of the entry, look also for the outlines of what was a 68-seat public latrine. Squatting atop one wing of the agora is the 17th-century Fetiye Mosque, now restored and housing a small but good exhibit on Hadrian's various innovations in Athens (including an olive oil tax, noted on a column by the agora entrance). The mosque interior frescoes have been lost – except for a tiny patch high on the back wall.
Admission to the site is included with the Acropolis combo ticket (€30), which permits entry to the Acropolis and six other sites (including this one) within five days.