Following the walls from south to north, the Golden Gate (Greek: Χρυσεία Πύλη, Chryseia Pylē; Latin: Porta Aurea; Turkish: Altınkapı or Yaldızlıkapı), is the first gate to be encountered. It was the main ceremonial entrance into the capital, used especially for the occasions of a triumphal entry of an emperor into the capital on the occasion of military victories or other state occasions such as coronations. On rare occasions, as a mark of honor, the entry through the gate was allowed to non-imperial visitors: papal legates (in 519 and 868) and, in 710, to Pope Constantine. The Gate was used for triumphal entries until the Komnenian period; thereafter, the only such occasion was the entry of Michael VIII Palaiologos into the city on 15 August 1261, after its reconquest from the Latins. With the progressive decline in Byzantium's military fortunes, the gates were walled up and reduced in size in the later Palaiologan period, and the complex converted into a citadel and refuge. The Golden Gate was emulated elsewhere, with several cities naming their principal entrance thus, for instance Thessaloniki (also known as the Vardar Gate) or Antioch (the Gate of Daphne), as well as the Kievan Rus', who built monumental "Golden Gates" at Kiev and Vladimir.
The date of the gate's construction is uncertain, with scholars divided between Theodosius I and Theodosius II. Earlier scholars favored the former, but the current majority view tends to the latter, meaning that the gate was constructed as an integral part of the Theodosian Walls. The debate has been carried over to a Latin inscription in metal letters, now lost, which stood above the doors and commemorated their gilding in celebration of the defeat of an unnamed usurper:
Haec loca Theudosius decorat post fata tyranni.
aurea saecla gerit qui portam construit auro.
Theodosius adorned these places after the downfall of the tyrant.
He brought a golden age who built the gate from gold.
Curiously, the legend has not been reported by any Byzantine author. However, an investigation of the surviving holes wherein the metal letters were riveted verified its accuracy. It also showed that the first line stood on the western face of the arch, while the second on the eastern. According to the current view, this refers to the usurper Joannes (r. 423–425), while according to the supporters of the traditional view, it indicates the gate's construction as a free-standing triumphal arch in 388–391 to commemorate the defeat of the usurper Magnus Maximus (r. 385–388), and which was only later incorporated into the Theodosian Walls.
The gate, built of large square blocks of polished white marble fitted together without cement, has the form of a triumphal arch with three arched gates, the middle one larger than the two others. The gate is flanked by large square towers, which form the 9th and 10th towers of the inner Theodosian wall. With the exception of the central portal, the gate remained open to everyday traffic. The structure was richly decorated with numerous statues, including a statue of Theodosius I on an elephant-drawn quadriga on top, echoing the Porta Triumphalis of Rome, which survived until it fell down in an earthquake in 740. Other sculptures were a large cross, which fell in an earthquake in 561 or 562; a Victory, which was cast down in the reign of Michael III; and a crowned Fortune of the City. In 965, Nikephoros II Phokas installed the captured bronze city gates of Mopsuestia in the place of the original ones.
The main gate itself was covered by an outer wall, pierced by a single gate, which in later centuries was flanked by an ensemble of reused marble reliefs. According to descriptions of Pierre Gilles and English travelers from the 17th century, these reliefs were arranged in two tiers, and featured mythological scenes, including the Labours of Hercules. These reliefs, lost since the 17th century with the exception of some fragments now in the Istanbul Archaeological Museum, were probably put in place in the 9th or 10th centuries to form the appearance of a triumphal gate. According to other descriptions, the outer gate was also topped by a statue of Victory, holding a crown.
Despite its ceremonial role, the Golden Gate was one of the stronger positions along the walls of the city, withstanding several attacks during the various sieges. With the addition of transverse walls on the peribolos between the inner and outer walls, it formed a virtually separate fortress. Its military value was recognized by John VI Kantakouzenos (r. 1347–1354), who records that it was virtually impregnable, capable of holding provisions for three years and defying the whole city if need be. He repaired the marble towers and garrisoned the fort with loyal Catalan soldiers, but had to surrender it to John V Palaiologos (r. 1341–1391) when he abdicated in 1354. John V undid Kantakouzenos' repairs and left it unguarded, but in 1389–90 he too rebuilt and expanded the fortress, erecting two towers behind the gate and extending a wall some 350 m to the sea walls, thus forming a separate fortified enceinte inside the city to serve as a final refuge. In the event, John V was soon after forced to flee there from a coup led by his grandson, John VII. The fort held out successfully in the subsequent siege that lasted several months, and in which cannons were possibly employed. In 1391 however, John V was compelled to raze the fort by Sultan Bayezid I (r. 1382–1402), who otherwise threatened to blind his son Manuel, whom he held captive. Emperor John VIII Palaiologos (r. 1425–1448) attempted to rebuild it in 1434, but was thwarted by Sultan Murad II.