As is written in Wiki, the building has a pronaos, a cella housing cult images at the centre of the structure, and an opisthodomos. The alignment of the antae of the pronaos with the third flank columns of the peristyle is a design element unique middle of the 5th century BC.
There is also an inner Doric colonnade with five columns on the north and south side and three across the end (with the corner columns counting twice). Both the pronaos and the opisthodomos are decorated with continuous Ionic friezes (instead of the more typical Doric triglyphs, supplementing the sculptures at the pediments and the metopes.
Only 18 of the 68 metopes of the temple of Hephaestus were sculptured, concentrated especially on the east side of the temple; the rest were perhaps painted. According to Pausanias, the temple housed the bronze statues of Athena and Hephaestus.