The original stoa was a gift, and a personal one. Attalos II ruled the kingdom of Pergamon, in what is now western Turkey, from 159 to 138 BC, but as a young man he had studied in Athens under the philosopher Carneades. Around 150 BC he repaid that education the way a wealthy monarch could - by building the city a covered marketplace grander than anything it had raised before. A dedicatory inscription cut into the architrave announced the donor's name to everyone who walked beneath it. Typical of the Hellenistic age, the stoa was larger and more elaborate than the older buildings around it, rising to two storeys when one was the norm. Its forty-two ground-floor rooms served as shops; the long colonnades, cool and shaded, became a promenade where Athenians strolled, traded, and argued. It was, in every practical sense, an ancient mall - and one financed by gratitude.

Look closely and the building reads like a textbook of Greek architecture, because it deliberately is one. The exterior ground-floor colonnade is Doric, plain and sturdy; the interior columns are Ionic, with their scrolled capitals. Climb to the upper floor and the grammar shifts again - Ionic outside, the ornate Pergamene order within. The whole thing is built of Pentelic marble and limestone, the same honey-colored stone that clad the Parthenon on the hill above. This layering of styles was no accident. It was a Hellenistic flourish, a way of showing command over the entire inherited vocabulary of the Greek world. Each storey carried two aisles and a row of rooms lit by small windows in the rear wall, with stairways at either end carrying visitors up to the second level.

By the twentieth century the stoa was little more than a footprint, its stone scattered and burned. Then, in 1948, the archaeologist Homer Thompson - field director of the Agora excavations - made an audacious proposal: rebuild it. Enough of the northern end survived for engineers to fix the exact original height, and the building's size suited it perfectly for a museum. The reconstruction ran from 1953 to 1956, carried out by the American School of Classical Studies at Athens. It was paid for by American donors, anchored by a one-million-dollar gift from John D. Rockefeller Jr., with further contributions from the industrialist Ward Canaday and his family. What rose was not a replica beside the ruins but the ruin itself, made whole - reassembled on its ancient foundations using its surviving members wherever they could be salvaged.

Today the Stoa of Attalos holds the Museum of the Ancient Agora, and its collection fits its setting. The objects here are the hardware of Athenian democracy: the kleroterion, a slotted stone machine used to randomly select citizen jurors; the ostraka, broken potsherds scratched with the names of men the assembly wished to exile - one bears the name of the general Cimon. There are bronze ballots, Geometric-era pottery from the eighth century BC, sculptures, coins, and a bust of the historian Herodotus. The history did not stop in antiquity, either. On 16 April 2003, the treaty admitting ten new nations - among them Cyprus, Poland, and the Baltic states - to the European Union was signed beneath this colonnade. In the building where Athenians once cast lots to govern themselves, modern Europe enlarged its own union.