Etrian Odyssey III: The Drowned City is a first-person dungeon crawl. It's the sort of game few developers make anymore, a distinction that allows it to stand out among the DS's library of RPGs.
Gameplay is split between exploring underground dungeons and sailing the waters around the complex. Your main job while exploring dungeons is to map each level out. You can draw walls, mark off paths or leave notes for yourself. The tools are incredibly simple to use, and I encourage you to map out every nook and cranny of the place. Each floor is huge (some can take an hour or more to explore), and you'll visit them multiple times. Floors are loaded with treasures, but you're going to need every bit of experience if you want to make it through to the next floor.
If The Drowned City has any flaws, it's the amount of repetition. There isn't much going on in terms of mechanics; the game sticks to a few well-designed components. It can get dull, though this sort of goes with the territory. Once you have a good idea of what the dungeon holds, you can always switch to "Auto." It takes some control from your hands, but helps speed up duller moments. Still, a few more quests wouldn't be a bad thing, if just to offer a goal while grinding.
Taking to the seas offers a similar, yet completely different experience. You're still looking for treasure and mapping areas out; though your travel time is limited by how much food you're carrying. It's a small, but important distinction. Aside from rare items and a few quests, the real reason to set sail is the money. It's probably one of the best ways to fill your coffers and equipment isn't cheap.
There's also a co-op component, though it's limited to battling bosses. It's not a total wash, but isn't as fully realized as it could have been.
The story never gets deeper than telling you to "just explore," which is just enough to get you started without bogging you down with long, drawn-out story sequences. There's a story here, though it's just enough to get your imagination's gears cranking; the rest of the story is really up to you. How do you tackle each dungeon? What monsters do you choose to go after? Much of what happens is based solely on your choices. The story asks for players to give themselves over to what's going on, but doing so makes for a more rewarding experience overall.