The architecture of Istanbul describes a large mixture of structures which reflect the many influences that have made an indelible mark in all districts of the city. The ancient part of the city (the historic peninsula) is still partially surrounded by the Walls of Constantinople, erected in the 5th century by Emperor Theodosius II to protect the city from invasion. The architecture inside the city proper contains buildings and structures which came from Byzantine, Genoese, Ottoman, and modern Turkish sources. The city has many architecturally significant entities. 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.
Ancient Greek and Roman structures
Serpent Column
One of the oldest monuments still surviving from antiquity is the Serpent Column, a monument built originally to honor Apollo for the victory over the Persians at Plataea in 479 BC. The column was moved by Constantine the Great when Constantinople became the new capital, and has stood at the Hippodrome of Constantinople ever since. According to legend, a member of the Polish Embassy visiting in 1700 damaged the top of it severely, breaking off the serpent heads, although in reality at least one of the heads had been reported as badly damaged two centuries earlier. The accepted version states that Mehmed II shattered it upon entering the city in triumph as its conqueror. The upper jaw of one of the snakes heads is on display at the Archaeological Museum in Istanbul.
Column of Constantine
The most important monuments of Roman architecture in the city include the Column of Constantine (), 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 in Gülhane Park, 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.
Byzantine and Genoese structures
Walls of Constantinople and environs
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 sea walls in the Seraglio Point area, which have continuously existed since Lygos and Byzantium, are the oldest part of the city walls; while the double land walls of Theodosius II at the western end of the city are the strongest parts. The northwestern section of the land walls was built in 627, during the reign of Heraclius (610–641), in order to accommodate the suburb of Blachernae, and added to by later emperors.
The city walls had 55 gates, the largest of which was the Porta Aurea (Golden Gate), the ceremonial entrance gate used by the Emperors, at the southwestern end of the triple land walls, close to the Sea of Marmara. Unlike the city walls which were built of brick and limestone, the Porta Aurea was built of large clean-cut white marble blocks in order to distinguish it from the rest, and a quadriga with elephant statues stood on its top. The doors of the Porta Aurea were made of gold, hence the name, which means "Golden Gate" in Latin.
In 1458, the Ottoman Sultan Mehmed II built the Yedikule Fortress to defend the Porta Aurea, which was incorporated into this castle and still stands as a part of the pentagon-shaped layout of the castle walls. The Column of Marcian () erected by Marcianus (reigned 450–457) dates from the same period as the triple land walls of Theodosius II.
The most extant Byzantine structure which has survived from the reign of Heraclius is the Prison of Anemas, incorporated into the city walls, at Blachernae. It is a huge castle-like structure with several towers and a network of underground Byzantine prisons.