Web of the Witch World (Witch World #2)

Classic Fantasy Series. "Simon Tregarth, whose own Earthly prowess had won him a throne and a witch-wife in an alien world, knew that both triumphs were precarious as long as the super-science of Kolder held a foothold on that planet. And his premonitions were right when those invaders from another dimension made their final diabolical strike for total conquest.