There is much difference of opinion as to the real date of construction of the present building. While some authorities have assigned it to the eleventh or even to the twelfth century, others have supposed that it is either a Christian temple constructed in the sixth century on the site of the old Temple of Mars, or the original Temple converted into Christian use. It has indeed been recently urged that it is essentially a genuine Roman work of the fourth century, very analogous in structure to the Pantheon at Rome, on the model of which it was probably built. The little apse to the south-west--the part which contains the choir and altar--is certainly of the twelfth century. There was originally a round opening at the centre of the dome--like the Pantheon--and under this opening, according to Villani, the statue of Mars stood. It was closed in the twelfth century. The dome served Brunelleschi as a model for the cupola of Santa Maria del Fiore. The lantern was added in the sixteenth century. Although this building, so sacrosanct to the Florentines, had been spared by the Goths and Lombards, it narrowly escaped destruction at the hands of the Tuscan Ghibellines. In 1249, when the Ghibellines, with the aid of the Emperor Frederick II., had expelled the Guelfs, the conquerors endeavoured to destroy the Baptistery by means of the tower called the Guardamorto, which stood in the Piazza towards the entrance of the Corso degli Adimari, and watched over the tombs of the dead citizens who were buried round San Giovanni. This device of making the tower fall upon the church failed. "As it pleased God," writes Villani, "through the reverence and miraculous power of the blessed John, the tower, when it fell, manifestly avoided the holy Church, and turned back and fell across the Piazza; whereat all the Florentines wondered, and the People greatly rejoiced." At the close of the thirteenth century, in those golden days of Dante's youth and early manhood, there were steps leading up to the church, and it was surrounded by these tombs. Many of the latter seem to have been old pagan sarcophagi adopted for use by the Florentine aristocracy. Here Guido Cavalcanti used to wander in his solitary musings and speculations--trying to find out that there was no God, as his friends charitably suggested--and Boccaccio tells a most delightful story of a friendly encounter between him and some young Florentine nobles, who objected to his unsociable habits. In 1293, Arnolfo di Cambio levelled the Piazza, removed the tombs, and plastered the pilasters in the angles of the octagonal with slabs of black and white marble of Prato, as now we see. The similar decoration of the eight faces of the church is much earlier.