The Iron Gate is a defile

Defile (geography)

Defile is a geographic term for a narrow pass or gorge between mountains or hills. It has its origins as a military description of a pass through which troops can march only in a narrow column or with a narrow front...

 between Balkh

Balkh

Balkh , was an ancient city and centre of Zoroastrianism in what is now northern Afghanistan. Today it is a small town in the province of Balkh, about 20 kilometers northwest of the provincial capital, Mazar-e Sharif, and some south of the Amu Darya. It was one of the major cities of Khorasan...

 and Samarkand

Samarkand

Although a Persian-speaking region, it was not united politically with Iran most of the times between the disintegration of the Seleucid Empire and the Arab conquest . In the 6th century it was within the domain of the Turkic kingdom of the Göktürks.At the start of the 8th century Samarkand came...

. It breaks up the mountains which extend from the Hisar range south towards the Amu Darya

Amu Darya

The Amu Darya , also called Oxus and Amu River, is a major river in Central Asia. It is formed by the junction of the Vakhsh and Panj rivers...

. In ancient times it was used as the passage between Bactria

Bactria

Bactria and also appears in the Zend Avesta as Bukhdi. It is the ancient name of a historical region located between south of the Amu Darya and west of the Indus River...

 and Sogdia and was likely of great importance to any power in the region. Its name comes from the belief that an actual gate, reinforced with Iron, stood in the defile.