The skyline of Rome is characterized by domes, rather than by bell towers. Most of the largest churches do not have a prominent bell tower: this is especially true for those built during the Renaissance and afterwards.
Some of the oldest Roman bell towers were built with a defensive aim:
both SS. Quattro Coronati and S. Saba stood in isolation at the top of a hill and were surrounded by walls: their
bell towers were thickly built and had very little decoration as if they belonged to a medieval castle,
rather than to a church.
The bell tower of S. Nicola in Carcere was originally part of the defensive system built by the Pierleoni family who controlled this part of Rome in the XIIth century.
During the XIIth and the XIIIth century the popes acquired greater authority and many old churches were enlarged or
rebuilt: their bell towers follow a common Romanesque pattern: they are marked into clearly defined stages by horizontal courses. The narrow windows show the influence of Byzantine architecture.
The bell tower of S. Maria del Popolo was built at a later period and is more similar to bell towers of northern Italy (another bell tower of this kind can be seen at S. Maria dell'Anima).
SS. Giovanni e Paolo
The bell tower of SS. Giovanni e Paolo is perhaps the finest Romanesque bell tower of Rome: the first two storeys (above the ancient Roman stone structure) were completed by 1118 and in 1150 the other five storeys were added. In 1190-1210 ca. the whole bell tower was decorated with porphyry slabs and Arab ceramics.
The bell tower of S. Francesca Romana was built approximately in the same period and in the same style and decoration as that of SS. Giovanni e Paolo. Other Romanesque bell towers were built at S. Alessio, S. Bartolomeo all'Isola, S. Benedetto in Piscinula, S. Cecilia, S. Crisogono, S. Croce in Gerusalemme, S. Eustachio, S. Giorgio in Velabro, S. Giovanni a Porta Latina, S. Gregorio Nazianzeno, S. Lorenzo in Lucina, S. Marco, S. Maria in Monticelli, S. Maria in Trastevere, S. Maria Maggiore, S. Pudenziana, SS. Rufina e Seconda, S. Sisto Vecchio and S. Spirito in Sassia. They are so many that one could say that the typical bell tower of Rome is a Romanesque one. Its design was always appreciated and when a church was largely restructured its Romanesque bell tower was not modified.
In 1251 the Franciscans started to build S. Maria in Aracoeli: the original
design included a bell tower, but in 1260 the Constitutiones Narbonensi, the rules established by the general chapter of the order in
the French town of Narbonne recommended that Franciscan churches should visually portray the modesty and poverty of the order: so the design of
S. Maria in Aracoeli had to be revised and the bells were placed in a simpler structure,
vaguely resembling a sail (in Italian it is called campanile a vela - sail bell tower).
This structure was adopted also in non Franciscan churches and in public buildings.
Gian Lorenzo Bernini designed two bell towers
for S. Pietro, but the project was abandoned
because they were too heavy for the once marshy ground near the Tiber: also his small bell towers for the Pantheon
must have been designed on an unlucky day because they were demolished in the XIXth century.
Francesco Borromini was more successful: the bell towers of S. Agnese in Agone are regarded as the finest ones of the Roman Baroque. His bell tower for S. Andrea delle Fratte explains why his detractors regarded his works as extravagant ones.
The architects who came after Borromini did not follow the example of S. Agnese in Agone and their
bell towers were in general minor additions to the design of the churches.