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One has to distinguish the things for which we have direct archaeological evidence, the things which can be reliably reconstructed from literature sources, and the things which are mentioned in the literature but with inadequate description such that we have no idea how they really looked.
To the first type belongs the Antikythera mechanism.
Examples of second kind are artillery pieces, and various machines described in the book of Heron.
Examples of the third kind are Archimedes planetarium and especially the enormous warships of Hellenistic era. See, for example, Leontophoros, Tessarakonteres.
All these things belong to the Hellenistic epoch.