The monumental Roman period gateway today known as "Hadrian's Gate" stood at the Triodos, the junction of three streets at the lower end of Kuretes Street, on the south side (the left side as you go down the street). It marked the start of the road northwestwards to Ortygia (see gallery page 17).
Originally over 16.6 metres tall and 11.4 metres wide but only around 1.68 metres deep, the gateway was made of white, grey-veined marble. It consisted of three storeys, the lowest of which had a tripartite entrance: a central archway, with a lower, narrower opening at each side of it. The arch itself spanned the centre of the second storey. There are thought to have been statues of deities, including Artemis, the imperial family and dignitaries between the columns of the upper levels.
Little of the monument has survived, but parts of the two narrower entrances have been reconstructed, each with part of an architrave supported at either side by a fluted column and a pillar with Corinthian capitals, both standing on a single pedestal.
It is thought that building began on the gate just before 117 AD, during the reign of Emperor Trajan (98-117 AD). On stylistic grounds it has been variously dated to some time between 113/114 and 127/128 AD, and the style of the decoration has been compared to that of other buildings such as the Library of Celsus and the "Temple of Hadrian". For example, like the "Temple of Hadrian", the pediment at the top of the gate had an inserted arch in a manner sometimes referred to as Syrian.
A fragmentary dedicatory inscription mentions an emperor, but his name has not survived. The building has been associated with Hadrian partly because of its similarity to the Gate of Hadrian in Athens, which may have been modelled on this and other similar gates built earlier in Anatolia (Asia Minor). There appears to be no direct evidence to support the theories that the gate was dedicated to Trajan or Hadrian during a visit by either emperor to Ephesus (for visits by Hadrian, see the note on gallery page 21).
Having collapsed in the late 3rd or 4th century AD, perhaps around 262/262 AD at the time of a major earthquake followed by the sack of Ephesus by the Goths, it was rebuilt in the 4th - 5th century AD as a fountain type structure with a water basin. In an inscription of the 5th century a Christian named Demeas boasts that he removed the statue of Artemis from the gateway and added a cross.  A cross was also carved on the arch, transforming the gate into a Christian monument.
Archaeologists unearthed parts of the gate around 1904, and subsequent excavations over the next 40 years in the surrounding area revealed other parts, many of which had been reused in other structures. The German architect and archaeologist Hilke Thür completed the theoretical restoration of the gateway in 1986, and the two outer entrances were reconstructed 1987-1994, begun by the Austrian architect and archaeologist Friedmund Hueber, with Thür in charge of the project from 1992.
See: Hilke Thür, Das Hadrianstor in Ephesos. In: Forschungen in Ephesos Band 11/1. Verlag der Österreichischen Akademie der Wissenschaften, Vienna, 1989.
Since the 1980s a number of the monuments in Ephesus have been reexamined and reappraised, sometimes leading to new ideas and theories concerning their history. However, it seems that no further investigation of "Hadrian's Gate" has been undertaken since the publication of Thür's book, which remains the most cited work on the monument.