The Arsenal out of Time by David McDaniel Ace G-667 1967

Back before mankind entered the realm of infinite space, back before Earth itself was anything but a primeval jungle, there were still stars that had inhabited planets and there had been a galactic civilization and an extra-galactic empire. And when millions of years later men ventured among the stars they found only the faintest traces of this primal civilization — until the moment when a space map was found giving the coordinates of an unknown world that had been the arsenal of the unimaginably mighty weapons of that First Empire. Then the quest was on — for whoever located that treasure trove first could dictate his own terms to a thousand proud planets.