The first endeavor to build a sanctuary for Athena Parthenos on the site of the present Parthenon was begun shortly after the Battle of Marathon (c. 490–488 BC) upon a muscular limestone foundation that extended and leveled the southern part of the Acropolis summit. This building replaced a hekatompedon (meaning "hundred-footer") and would have stood beside the archaic temple dedicated to Athena Polias ("of the city"). The Older or Pre-Parthenon, as it is frequently referred to, was still under construction when the Persians sacked the city in 480 BC and razed the Acropolis. In the mid-5th century BC, when the Athenian Acropolis became the seat of the Delian League and Athens was the greatest cultural centre of its time, Pericles initiated an ambitious building project that lasted the entire second half of the century. The most important buildings visible on the Acropolis today—the Parthenon, the Propylaia, the Erechtheion and the temple of Athena Nike—were erected during this period. The Parthenon was built under the general supervision of the artist Phidias, who also had charge of the sculptural decoration. The architects, Ictinos and Callicrates, began in 447 BC, and the building was substantially completed by 432, but work on the decorations continued until at least 431. Some of the financial accounts for the Parthenon survive and show that the largest single expense was transporting the stone from Mount Pentelicus, about 16 kilometres from Athens, to the Acropolis. The funds were partly drawn from the treasury of the Delian League, which was moved from the Panhellenic sanctuary at Delos to the Acropolis in 454 BC. Although the nearby Temple of Hephaestus is the most complete surviving example of a Doric order temple, the Parthenon, in its day, was regarded as the finest. The temple, wrote John Julius Cooper, "enjoys the reputation of being the most perfect Doric temple ever built. Even in antiquity, its architectural refinements were legendary, especially the subtle correspondence between the curvature of the stylobate, the taper of the naos walls and the entasis of the columns." Entasis refers to the slight diminution in diameter of the columns as they rise, though the observable effect on the Parthenon is considerably more subtle than on earlier temples. The stylobate is the platform on which the columns stand. As in many other classical Greek temples, it has a slight parabolic upward curvature intended to shed rainwater and reinforce the building against earthquakes. The columns might therefore be supposed to lean outwards, but they actually lean slightly inwards so that if they carried on, they would meet almost exactly a mile above the centre of the Parthenon; since they are all the same height, the curvature of the outer stylobate edge is transmitted to the architrave and roof above: "All follow the rule of being built to delicate curves," Gorham Stevens observed when pointing out that, in addition, the west front was built at a slightly higher level than that of the east front. It is not universally agreed what the intended effect of these "optical refinements" was; they may serve as a sort of "reverse optical illusion". As the Greeks may have been aware, two parallel lines appear to bow, or curve outward, when intersected by converging lines. In this case, the ceiling and floor of the temple may seem to bow in the presence of the surrounding angles of the building. Striving for perfection, the designers may have added these curves, compensating for the illusion by creating their own curves, thus negating this effect and allowing the temple to be seen as they intended. It is also suggested that it was to enliven what might have appeared an inert mass in the case of a building without curves, but the comparison ought to be with the Parthenon's more obviously curved predecessors than with a notional rectilinear temple. Some studies of the Acropolis, including the Parthenon, conclude that many of its proportions approximate the golden ratio. The Parthenon's façade as well as elements of its façade and elsewhere can be circumscribed by golden rectangles. This view that the golden ratio was employed in the design has been disputed in more recent studies. Measured at the stylobate, the dimensions of the base of the Parthenon are 69.5 metres by 30.9 metres (228.0 x 101.4 ft). The cella was 29.8 metres long by 19.2 metres wide (97.8 x 63.0 ft), with internal colonnades in two tiers, structurally necessary to support the roof. On the exterior, the Doric columns measure 1.9 metres (6.2 ft) in diameter and are 10.4 metres (34.1 ft) high. The corner columns are slightly larger in diameter. The Parthenon had 46 outer pillars and 23 inner pillars in total. The stylobate has an upward curvature towards its centre of 60 millimetres (2.36 in) on the east and west ends, and of 110 millimetres (4.33 in) on the sides. The roof was covered with large overlapping marble tiles known as imbrices and tegulae.