A wall assembly for forming an external wall on a building structure is disclosed. The wall assembly comprises a cladding and a plurality of vertical formwork members spaced along the wall assembly, and a horizontal formwork member extending along the top of the wall assembly. The formwork members are elongate profiles having a substantially constant cross sectional shape and they are arranged so that with the cladding they form an interconnected formwork defining a formwork space. A cementitious material cast within the formwork space is interconnected and forms a single underlying structural frame within the constructed wall assembly. The assembly includes connectors extending between the vertical and horizontal formwork members and the cladding on which they are mounted. Each connector extends from the cladding through the formwork space to the formwork member and is cast within the cementitious material. A method of building a wall assembly is also disclosed.