This Doric peristyle temple, constructed around 550 BC, is one of the oldest standing temples in Greece.
The columns are made of poros or limestone stone; there were six in front and rear and 15 on both sides. They used to be covered with stucco, and were can see some traces of it. The columns are monolithic, and ca. 7 m high, and 1.8 m in diametre at the base. A colum has 20 vertical flutes. The large and flat capitals are characteristic of the middle 6th-century Doric temples.
Now only seven columns are standing. The Roman colonists removed the interior columns to reuse in the construction of the south-west portico in the Forum. As they also modified the structure of cellae, we don't know how the inside was originally arranged.Reference