This photo was taken from the duomo in Pisa, somewhere in the back along the exterior wall of the apse. Begun in 1064, construction on the city’s cathedral must have been an ongoing project that would evolve over the centuries. The Pisans must not have been shy about re-using older building material, which would explain how this—and other—stones with ancient Roman inscriptions ended up on view.
What puzzles me is that they didn’t face the stone, or at least turn it around. Don’t get me wrong, I’m grateful that the masons responsible left the stone as is, otherwise we’d never get this tasty treat. My gratefulness does not lead me to better understanding, however. Doesn’t reusing old artifacts in this manner destroy the visual harmony of the building? The cathedral is quite an impressing building, even if it doesn’t lean as much as its bell tower.