Located opposite the Parthenon, the Erechtheion is one of the most distinctive buildings standing on the Athenian Acropolis constructed between 421 and 406 BC. The Erechtheion replaced the Old Temple of Acropolis, which was destroyed by the Persians in 480 BC. The asymmetrical building was built of Pentelic marble, with friezes of black Elusinian limestone to take applied white marble relief sculpture. ( "Erechtheum (Erechtheion)" ) Since the complex temple sits on a slopping site, it has an irregular floor plan which consists of four chambers with multiple functions. It housed a great variety of ancient cults and many sacred objects, including the venerable image of Athena Polias in the east cella, a golden lamp made by Callimachus, a well containing sea water, which is known as the Erechtheian Sea, and the mark of Poseidon's trident, sacred shrines like the altar of Poseidon, and the altars of Boutes and Hephaestus. (Roberts, "Erechthēum" ) To the south-west of the building stood the olive tree, which is a gift of Athena to the city of Athens.
The whole structure also consists of two porches, the north porch at the northwest corner is supported by six tall Ionic columns, stands at a lower level and gives access to the western cella, while below its floor it was believed to be the spot where Zeus killed the legendary King Erechteus with a thunderbolt. (theacropolismuseum.gr) In the south-west corner, there stands a uniquely projecting porch which is the most well-known part of the Erechtheion. It is supported by six massive female statues, and hence named the Porch of the Maidens, with the supporting figures known as caryatids. Below it stood the grave of Kekrops, another legendary King of Athens. (theacropolismuseum.gr) The second caryatids from the western section in the front row was removed by Lord Elgin in 1801 and relocated to the British Museum, while the rest of the original caryatids are being displayed at the Acropolis Museum in Athens. All caryatids are now replaced by exact replicas of the originals. ("Erechtheum") So, what is the significance of the caryatids at the Erechtheion?
There is no definite explanation for why the maidens were used, and there are different interpretations regarding the reasons behind the construction of caryatids instead of regular Ionic columns that can be found at different parts of the building. Below is an account of the origin of Caryatids that was considered as necessary towards the education of architects, written by the first century roman writer, Vitruvius:
'But he must acquaint himself with many narratives from history; for architects often incorporate many ornamental features in the designs of their works, of which they must be able to give a reasoned account, when asked why they added them. For example, if any one erects marble statues of robed women, which are called Caryatids, instead of columns on his building, and places mutules and crowning members above them, this is how he will explain...