If you look closely, the lowest tier of tiles appears to dip slightly inward and downward from the left and right, and then forms a somewhat straight line at the center. This is how the eye (and the lens) sees things. However, this is an optical illusion which would be seen from any viewpoint. The tiles are, like the top two tiers, in a circle and with no dip at all; they are all on the same level. The architects and engineers of the Parthenon would approve.