The game is about defending a town played out in two phases, day and night. When the sun is in view, players recruit adventurers who head out for materials and build defenses to prevent monsters from attacking at night. So where do the dungeons or dams come into play? well while the beasts attack you need to direct flowing water to either help you or bring destruction using dams, and do battle with your attackers.
It sounds like it could be nice time killer for the DS, but we need to see footage really : /