The Tholos of Athena Pronaia
It is the most remarkable and unusual monument while being a masterpiece of Classical Architecture, in Delphi.
The Tholos is a circular building and is the first visual part of the sanctuary seen before a visitor reaches the Temple of Apollo, thus acquiring the name Pronaia (before the temple). It was important because before a visitor could ask the oracle, they had to offer a sacrifice at the Athena Pronaia. Built in 380 B.C and designed by Theodoros of Fokea, with its exact function unknown, was constructed with 20 Doric Style columns around its external diameter and 10 Corinthian style columns in the interior.