Yup, that is it: the altar of the twelve gods. Not much to look at, admittedly.

But this is a significant site. It was the "zero milestone" that was used as the starting point of distance measurements.

The altar was discovered in the early 20th century. It is under the supporting structure for the subway line (the wall at the back of the photo). Despite Wikipedia claiming the structure was only found in 2011, the above photo was taken in 2009.

The twelve gods are: Zeus, Hera, Poseidon, Demeter, Hestia, Apollo, Artemis, Hephaestus, Athena, Ares, Aphrodite and Hermes.