The monument is known for its peculiar design: large defensive walls with embedded rooms and arches, with towers at regular intervals, enclosing a vast empty interior of 190 hectares (470 acres).

The massive walls contained a wealth of embedded structures: a row of around 828 rooms (circa 12 square metres (130 sq ft) in size) and hundreds of monumental arches, evoking royal Sasanian architecture.

[3][4][5] Popular beliefs identify the structure with the Citadel of Varena mentioned in the holy Zoroastrian text Vendidad.

[3] Qaleh Iraj is the largest pre-modern, non-urban permanent fortification in South-west Asia, Europe and Africa.

Another example is Qaleh Kharabeh near the Great Wall of Gorgan which had accommodated two neatly aligned rows of tents with wide corridors in between, suggesting similar role of the interior space in other bases.

[3] Qaleh Iraj was probably built in response to northern nomadic invaders which, on some occasions, penetrated deep into the Sasanian territory.

Iraj Castle is considered as the world's largest mud-brick fort with a height of 50 feet and an area of 200 hectares.