What stood above the foundations found in the center of the Baptistery, where we see an area of the floor devoid of marbles?
The prevailing interpretation is that there was a baptismal font, but this idea has no sense in a construction point of view: those foundations would be extremely oversized to support a very light weight. The great architect of the Temple-Baptistery could not make such gross errors that not even the last of his workers could make. This idea, in short, is an insult to his skillness; and let us not speak of other even more wrong ideas.

But if we look at those foundations as made to support the column of the ‘statue of Mars’ and its aedicule, everything would find a constructive logic and perfect proportions.
Indeed, these foundations suggest not a baptismal font but a trophaeic aedicule; and curiosity urges us to try to imagine how this aedicule could have been made.

I tried to do that, also following the image painted by Vasari in Palazzo Vecchio to represent the Temple of Mars in its original form. But if Vasari made a mistake because the Temple originally could not have been the form he imagined, that image is instead perfectly congruent with the foundations mentioned above, but which he could not know at his time. Is it a case? Or did Vasari have some document from which he drew those forms? Or perhaps Borghini suggested them to him on the basis of a document he had?
We don’t know.

Anyway, the foundations can give us some suggestions, as we see in this a fascinating image.

At the center of the Temple of Mars there could have been a trophaeic aedicule not very different from this image. The column inside it was the same that today we see in Piazza della Repubblica. The wall of the scarsella is not represented because at the time there was the entrance from the main street of the town, the cardo maximus of the Roman Florentia.
The central foundations as they would appear if they were visible. In this sketch they have been raised about 3 ft from the floor because surely the original aedicule was raised a few steps just as we see in Vasari’s painting.
The foundations of the hypothesized font would have had grossly incorrect dimensions: those of the balustrade (1) would have the same width as those supporting the columns (2), and those of the plinth (4) would be even wider than those of the perimeter walls supporting the dome (3).
Plans of the excavations and of the ground floor. The central foundations have the form of an octagonal ring and a square plinth.

The baptismal font as it was imagined in 1921 on the occasion of Dante’s celebrations.

The medieval baptismal font of San Giovanni had to be similar to the one of the Baptistery of Pisa, with a weight undoubtedly very modest. Note the little wells, one of which was broken by Dante to save a child who was drowning there. The officiant was in the center and administered baptism, passing in succession from one well to another.
This first attempt of mine to reconstruct the possible form of the trophaeic aedicule, constituted by only columns, had some incongruities with the findings of the foundations, for which subsequently I had to correct it.
Detail of the painting of the foundation of Florence by Vasari and Stradano in the ceiling of the Salone dei Cinquecento in Palazzo Vecchio with the original Temple of Mars represented. This image is not congruent with the actual building but with the central foundations found in the Baptistery. The aedicule of the trophy could have a shape very similar to this one.