Nostalgic, deep, compelling, challenging, and all in all, a fantastic game!
Upon entering the dungeon, you get a glimpse at what to expect for the rest of the game. The game is essentially a tile-based 3D explorer, allowing you to go step-turn-step-step-etc. The top screen gives you the graphical view, which for the DS, are honestly beautiful. The bottom screen gives you the map, which is where the uniqueness lies.
As you walk, you fill in the floor of a map. But to put in details, like doors, walls, treasure chests, warps, item collection points, etc., you have to draw them yourself. Doing so is the basis of your first mission, just to give you some experience.
Combat in the game is about what you'd expect - pseudo-random encounters (there's a little meter that tells you when to expect one) that bring you to a combat interface. You queue up your actions, and then allow them to execute. Next turn you queue up again.
And this is where the game gets *hard*. The difficulty curve on combat is steep - the critters on each level are anywhere from 2-3x tougher than the ones on the level above, and it's easy to get in over your head, especially with the special combats (called FOEs) that can be ridiculously tough. In fact, when getting to level 3, they even warn you that you can (and sometimes should!) avoid FOEs entirely, because they might be too tough for you - thankfully you can see them on the map. That can be really troublesome, because when you get into combat the FOEs can move in the open map after each turn, which means you might end up with one interrupting a random encounter if you're not careful.
Difficulty aside, Entrian Odyssey is deep, challenging, and incredibly fun. It's one of those Atlus masterpieces you'll regret not picking up earlier when it becomes an impossible-to-find gem.