It was the facade of the main palace in Cte­siphon, and is the only vis­i­ble remain­ing struc­ture of the ancient cap­i­tal city. The arch­way is con­sid­ered a land­mark in the his­to­ry of archi­tec­ture and is the sec­ond largest sin­­gle-span vault of unre­in­forced brick­work in the world.