Wolfe and the town's protector were faced with a conundrum. The spell could very well target them again, and they would stand little chance of winning without outside intervention, which would lead to the looting of their advanced technologies by the invaders.
The obvious answer was to hide all the military weapons in such a way that they wouldn't easily be found, but there was no way to tell what sort of items would be useful to an otherwise primitive magical society.
But more concerning was the fact that they might not be the first or the last target of the spell. If the off world humans were casting that spell regularly to create training areas, they could already have all sorts of military technology from this world, and nobody would know.
The spell would return everything to its original state, and then they would leave with their ill-gotten gains.
