 Old St. Peter's Basilica was the 4th-century church begun by the Emperor Constantine the Great between 319 and 333 AD.It was of typical basilical form, a wide nave and two aisles on each side and an apsidal end, with the addition of a transept or bema, giving the building the shape of a tau cross.It was over long, and the entrance was preceded by a large colonnaded atrium.