The twelve gods we're talking about here are Zeus, Hera, Poseidon, Demeter, Hestia, Apollo, Artemis, Hephaestus, Athena, Ares, Aphrodite and Hermes.
In ancient Athens, in the middle of the agora, was an altar dedicated to the whole crew. The Altar of the Twelve Gods was considered the centre of the city. Here's Nico describing the agora. I wrote this in my first book:
The open space in the middle was covered in a jumble of stalls, each little more than a rough plank resting upon a barrel at each end, with perhaps an awning to keep the vendors and their goods in the shade. I walked past the many stalls selling produce from the farms. These stalls were covered with jars and baskets of olives, olive oil, figs and grapes, corn, goat’s cheese and, rarely, smoked goat meat. Behind every stall stood a farmer, his skin leathery and dark from years working in the sun, his hands calloused, wearing rough clothes and a floppy sheepskin hat, shouting his products or dealing with a customer. These weren’t men to care much of politics; it was all they could do to scratch a living from the stony soil.
Barely visible, fenced off from the chaos, was the Altar of the Twelve Gods. The altar was the very center of Athens, the point from which all distances are measured. It was the only place in Athens dedicated to all twelve Gods, and so especially sacred: a place of sanctuary for anyone who could make it inside the fence before their pursuers reached them. The altar stone was made of marble, flat on top, and somewhat weathered though it had been set in place only sixty years before.When I wrote that, no one knew what the for real altar looked like, so I made something up. If the archaeologists win the battle to dig the location, this promises to be the first time a later discovery shows something I wrote to be wrong, or, if I'm incredibly lucky, right.