Main article: Architecture of Istanbul
Throughout its long history, Istanbul has acquired a reputation for being a cultural and ethnic melting pot. As a result, there are many historical mosques, churches, synagogues, palaces, castles and towers to visit in the city. Some of these historical structures, which draw millions to the city every year, reflect the heart and soul of Istanbul.
The famous Maiden's (Leander's) Tower, one of the symbols of Istanbul, was originally built by the ancient Athenian general Alcibiades in 408 BC to control the movements of the Persian ships in the Bosphorus strait. Back then it was located between the cities of Byzantion and Chrysopolis. The tower was later enlarged and rebuilt as a fortress by the Byzantine emperor Alexius Comnenus in 1110, and was rebuilt and restored several times by the Ottoman Turks, most significantly in 1509 and 1763.
The most important monuments of Roman architecture in the city include the Column of Constantine (Turkish: Çemberlitaş), which was erected in 330 by Constantine the Great for marking the declaration of the new capital city of the Roman Empire and contained several fragments of the True Cross and other artifacts belonging to Jesus Christ and Virgin Mary, the Mazulkemer Aqueduct, the Valens Aqueduct, the Column of the Goths at the Seraglio Point, the Milion which served for calculating the distances between Constantinople and other cities of the Roman Empire, and the Hippodrome of Constantinople which was built following the model of the Circus Maximus in Rome. Construction of the Walls of Constantinople began under Constantine the Great, who enlarged the previously existing walls of Byzantium in order to defend the new Roman capital city which quickly grew following its proclamation as Nova Roma. A new set of walls was built further west during the reign of Theodosius II, and rebuilt after an earthquake in 447 in their current shape.
The early Byzantine architecture followed the classical Roman model of domes and arches, but further improved these architectural concepts, as evidenced with the Hagia Sophia, which is the largest structure on Sultanahmet Square in the Eminönü district. The Hagia Sophia was designed by Isidorus and Anthemius as the third church to rise on this location, between 532 and 537, following the Nika riots (532) during which the second church was destroyed (the first church, known as the Megala Ekklessia ("Great Church") was inaugurated by Constantius II in 360; the second church was inaugurated by Theodosius II in 405, while the third and current one was inaugurated by Justinian in 537). The Church of Saints Sergius and Bacchus (commonly known as the Little Hagia Sophia), which was the first church built by Justinian in Constantinople and edificed between 527 and 536, had earlier signaled such an improvement in the design of domed buildings, which require complex solutions for carrying the structure. The present-day Hagia Irene (which was originally built by Constantine in the 4th century, but was later enlarged by Justinian in the 6th century) and the Basilica Cistern are also from this period.
The most important churches which were built after the Byzantines recovered Constantinople from the Latin Crusaders in 1261 include the Pammakaristos Church and Chora Church. Also in this period, the Genoese Podestà of Galata, Montano de Marinis, built the Palazzo del Comune (1314), a copy of the San Giorgio Palace in Genoa, which still stands in ruins on the back streets of Bankalar Caddesi in Galata, together with its adjacent buildings and numerous Genoese houses from the early 1300s. The Genoese also built the Galata Tower, which they named as Christea Turris (Tower of Christ), at the highest point of the citadel of Galata, in 1348.
The Ottoman Turks built the Anadoluhisarı on the Asian side of the Bosphorus in 1394, and the Rumelihisarı at the opposite (European) shore, in 1452, a year before the conquest of Constantinople. The main purpose of these castles, armed with the long range Balyemez (Faule Metze) cannons, was to block the sea traffic of the Bosphorus and prevent the support ships from the Genoese colonies on the Black Sea ports, such as Caffa, Sinop, and Amasra, from reaching Constantinople and helping the Byzantines during the Turkish siege of the city.
Following the Ottoman conquest of the city, Sultan Mehmed II initiated a wide scale reconstruction plan, which included the construction of grand buildings such as the Eyüp Sultan Mosque, Fatih Mosque, Topkapı Palace, the Grand Bazaar and the Yedikule (Seven Towers) Castle which guarded the main entrance gate of the city, the Porta Aurea (Golden Gate). In the centuries following Mehmed II, many new important buildings, such as the Süleymaniye Mosque, Sultanahmet Mosque, Yeni Mosque and numerous others were constructed. In the 18th and 19th centuries, traditional Ottoman architectural styles were gradually replaced by European styles, such as the Baroque style interiors of the Aynalıkavak Palace (16771679) and Nuruosmaniye Mosque (17481755, the first Baroque style mosque in the city, also famous for its Baroque fountain), and the 18th century Baroque additions to the Harem section of the Topkapı Palace. Following the Tanzimat reforms which effectively started Turkey's Europeanization process in 1839, new palaces and mosques were built in Neoclassical, Baroque and Rococo styles, or a mixture of all three, such as the Dolmabahçe Palace, Beylerbeyi Palace and Ortaköy (Mecidiye) Mosque.
Starting from the early 19th century, the areas around İstiklal Avenue were filled with grandiose embassy buildings belonging to prominent European states, and rows of European (mostly Neoclassical and later Art Nouveau) style buildings started to appear on both flanks of the avenue. Istanbul especially became a major center of the Art Nouveau movement in the late 19th and early 20th centuries, with famous architects of this style like Raimondo D'Aronco building many palaces and mansions in the city proper and on the Princes' Islands. His most important works in the city include several buildings of the Yıldız Palace complex, and the Botter House on İstiklal Avenue. The famous Camondo Stairs on Bankalar Caddesi (Banks Street) in Karaköy (Galata) is also a beautiful example of Art Nouveau architecture. Other important examples are the Hıdiv Kasrı (Khedive Palace) on the Asian side of the Bosphorus, Flora Han in Sirkeci, and Frej Apartmanı in the Şişhane quarter of Beyoğlu.