As the adventurers hide in the trees just off the riverbank, the river galley pulls abreast of them. The boat has a single sail and small cabins fore and aft. Amidships is an open deck with five large oars on each side. The tops of the heads of the oarsmen are just visible over the gunnel. The prisoners and horses are no longer on deck (but the aft cabin looks large enough to hold the horses). There are two men on the platform that makes the roof of the fore cabin and two on the aft cabin. The faint sound of a drum reaches the adventurers, in time with the stroke of the oars.

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I hope this was consistent with my earlier description. I'm calling it a "River Galley" to distinguish it from the much larger Galley in the DMG.