The core mechanic is borrowed from Snake, the old mobile game where you send a long snake around in four directions while eating bits of food and growing longer. In Nimble Quest, you guide around a train of fantasy-based adventurers by swiping on the screen, earning XP and gems as you go.
The gameplay is simple, but it can be tough. Running into a wall kills your whole party in one hit, and the adventurers you pick up during play (which are unlocked at a rate of about one per stage) are relatively fragile. You'll probably lose just about as many as you gain over the course of a level or game.
Just like in Pocket Planes and Tiny Tower, NimbleBit keeps the rewards flowing. Powerups boost your strength or give you a gem magnet, and clearing out an entire level is a very rewarding feat by itself. The game is freemium, but NimbleBit knows how to keep a freemium game fun even for those who don't want to spend money, so the in-app purchases are always there and ready if you'd like a hand, but never required or annoying.
The one misstep is that NimbleBit has borrowed the idea of "boosts." It's a concept that's ascribed to Bejeweled Blitz and has been showing up in several games lately. I don't like boosts at all. The idea is that you can spend some money on unlocking an extra ability or some extra power for a certain stage, essentially gambling that you'll earn just as much or more back with that extra power. In my experience, buying the boosts is never worth it, so I just don't use them at all, and that seems like a less-than-ideal mechanic in a game that's otherwise so polished.
That quibble aside, Nimble Quest is another gem in NimbleBit's crown. It'll be interesting to see what kind of audience it finds, as it's a very different game from the others the Marsh brothers are known for. But it's definitely a must-play, and if you're anything like me, this game will snake its way right into your free time for quite a while.