Planet of Peril by Otis Adelbert Kline — Read

The pandemonium of battle was punctuated by the staccato reports of the men's cannonlike mattorks as Grandon reached the interior of the men's sleeping quarters. He mounted to the topmost chamber from which Joto directed the activities of the two mattork crews while he shouted orders to the defenders.

Here he found Vernia and Rotha. "See," the marsh-girl cried, "they come by thousands and tens of thousands. They cover the entire landscape. Our defenses will be crushed."