The Tower of the Winds in Athens figures among the most ingenious creations of ancient architecture. Erected at the end of the 2nd cent. A. D. it is the best-preserved building from antiquity in all of Greece and the only one with a completely extant roof. The tower has an octagonal floor plan and consists from bottom to top of fine Pentelic marble. The truly unique roofing is constructed with 24 tapering solid slabs abutting a central keystone. On the outside these slabs create an octagonal pyramid, whilst on the inside they have a round base and form a dome-like ceiling. The slabs rest on the walls of the tower and are kept in place by the ring of the sima blocks. In order to sustain the considerable pressure, these blocks are carefully linked together with iron-clamps. The roofing has a serious weight with a span of more than seven meters - for its period a rather daring construction.