Taq Kasra is a giant iwan (vaulted hall open at one end), the only major surviving fragment of Ctesiphon, capital of the Sasanian Empire — and on its own it draws the limit of how far the brick vault can go. Where the Roman groin vault relies on concrete and temporary timber formwork (centring), the Sasanian builders find a way without stone, without concrete, even without formwork.

The problem is this: while raising a vault of nearly 25 metres' span, how are the bricks held aloft before the mortar has set? The answer lies in the vault's shape and in the laying technique. The vault is not a semicircle but a parabolic (near-catenary) curve tapering upward; this form keeps the material almost entirely in compression, reducing the weak points that produce sideways thrust. The bricks are laid not vertically but in inclined rings leaning slightly against the back wall; each new ring adheres to the last and carries itself, so no timber centring need be built beneath. Fast-setting gypsum mortar makes this possible.

The result is one of the boldest single-span arches of the ancient world: a vast portal open to the sky, rising from thick mud-brick walls. Taq Kasra represents a branch of the dome-and-arch tradition outside Rome — Iran's brick-and-iwan architecture; this line will pass directly into the iwan courtyards and the pointed arch of later Islamic architecture.