<P> 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 103.6 metres (340 ft) long, and the entrance was preceded by a large colonnaded atrium . This church had been built over the small shrine believed to mark the burial place of St. Peter . It contained a very large number of burials and memorials, including those of most of the popes from St. Peter to the 15th century . Like all of the earliest churches in Rome, both this church and its successor had the entrance to the east and the apse at the west end of the building . Since the construction of the current basilica, the name Old St. Peter's Basilica has been used for its predecessor to distinguish the two buildings . </P> <P> By the end of the 15th century, having been neglected during the period of the Avignon Papacy, the old basilica had fallen into disrepair . It appears that the first pope to consider rebuilding, or at least making radical changes was Pope Nicholas V (1447--55). He commissioned work on the old building from Leone Battista Alberti and Bernardo Rossellino and also had Rossellino design a plan for an entirely new basilica, or an extreme modification of the old . His reign was frustrated by political problems and when he died, little had been achieved . He had, however, ordered the demolition of the Colosseum and by the time of his death, 2,522 cartloads of stone had been transported for use in the new building . The foundations were completed for a new transept and choir to form a domed Latin cross with the preserved nave and side aisles of the old basilica . Some walls for the choir had also been built . </P> <P> Pope Julius II planned far more for St Peter's than Nicholas V's program of repair or modification . Julius was at that time planning his own tomb, which was to be designed and adorned with sculpture by Michelangelo and placed within St Peter's . In 1505 Julius made a decision to demolish the ancient basilica and replace it with a monumental structure to house his enormous tomb and "aggrandize himself in the popular imagination". A competition was held, and a number of the designs have survived at the Uffizi Gallery . A succession of popes and architects followed in the next 120 years, their combined efforts resulting in the present building . The scheme begun by Julius II continued through the reigns of Leo X (1513--1521), Hadrian VI (1522--1523). Clement VII (1523--1534), Paul III (1534--1549), Julius III (1550--1555), Marcellus II (1555), Paul IV (1555--1559), Pius IV (1559--1565), Pius V (saint) (1565--1572), Gregory XIII (1572--1585), Sixtus V (1585--1590), Urban VII (1590), Gregory XIV (1590--1591), Innocent IX (1591), Clement VIII (1592--1605), Leo XI (1605), Paul V (1605--1621), Gregory XV (1621--1623), Urban VIII (1623--1644) and Innocent X (1644--1655). </P> <P> One method employed to finance the building of St. Peter's Basilica was the granting of indulgences in return for contributions . A major promoter of this method of fund - raising was Albrecht, Archbishop of Mainz and Magdeburg, who had to clear debts owed to the Roman Curia by contributing to the rebuilding program . To facilitate this, he appointed the German Dominican preacher Johann Tetzel, whose salesmanship provoked a scandal . </P>

Who designed the dome of st peter's basilica