The right way to build the Archimedes Death Ray is of course solar panels charging energy storage, then powering laser guns.
Tell a fanciful tale that that is what Archimedes originally designed, and he even built and demonstrated a working prototype. However, in then an even more fanciful conversation with the local political leader, they discuss in detail the quantum mechanics underlying the function of many parts of the device -- the local political leader is intelligent enough to understand and discuss quantum mechanics -- and they both agree that quantum mechanics is witchcraft and its powers were not meant for man to wield, so they disassemble the prototype and switch to plan B involving polished mirrors.
Similarly, they also decide not to use nuclear weapons against the Romans.
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