400 Years* is a game about waiting.
Hold up, hold up, it actually makes waiting interesting.
(Recommended listening: Some manner of loop of Fall Out Boy's "Immortals." This includes while playing the game. It's actually astoundingly appropriate.)
If you come into the game blind, it looks kind of like a standard platformer where you play as a rather Moai-esque stone being. There's one key difference, though:
You CANNOT. JUMP. EVER. Instead, you must use patience, the changes of the seasons, and the passage of time to overcome the obstacles that face you. Case in point: You frequently must pass water by waiting for winter.
Yeah, you cross water by waiting for it to turn to ice. It gets better: If you're in the water when it freezes over, it (usually) causes you to freeze too.
(Exceptions occur in certain places where you're sufficiently deep; if you're sorta deep, you just get sorta stuck; if you're really deep, you can actually, in the game's only bug I really found, phase through the ice to the surface magically.)
If you press the space bar briefly, it only moves time forward a little bit, but if you hold it down, time passes super fast and the screen has a neat effect.
Also pictured above: You can wait for trees to grow. You can even plant chestnuts so that new trees grow. And then you can climb them. It's pretty neat, and it's not even the only form of time and effort investment that gives you the ability to pass difficult obstacles. (There are also non-useful trees that grow, die, and are replaced with new trees. And the seasons change the trees. There's a lot of nice little touches.)
But, as the title suggests, you "only" have four hundred years, because the whole reason this stone guy got rolling (...sorry) in the first place is because he foresaw a cataclysm that would happen in that length of time, and he's set out to stop it.
If you wait for a full four hundred years...
EVERYBODY DIES. Except probably the stone guy; he seems pretty tough, a lot like Great Ag and Ob, right down to the waiting around forever for stuff to happen, though he also seems a lot less interested in conquest. (Kudos to my sister for insisting on this, although I'd have gotten around to it myself. She was the one that held the space bar down for it, though.)
So what do you need to do to finish the game? Well, honestly, it's not especially hard; even on my first playthrough it took me "only" 202 years to reach the end (and I definitely made a few significant mistakes), and the volcano that the stone man plugs with his body to prevent the world-ending eruption. (Slightly relevant: Volcano-plugging from a Golden Age comic.)
And if you wait, you discover that apparently, the stone man's journey started in about 1613, because when you press space to wait while plugging the volcano, it takes you to this ending sequence with a jet plane inexplicably peacefully flying peacefully past.
This is arguably a case of self-sacrifice (although actually plugging a volcano is the silliest idea), but let's be realistic here. He's an earth elemental who's calming it with his powers. Or an indestructible stone alien like Ag and Ob (who are, if you didn't bother reading the old post, only stunned by nukes). He's probably fine.
Anyway, this is a neat little game and quite a bit more fun than you'd expect waiting around to be.
*I've unfortunately seen this game be mislabeled "Time Stands Still" by the Tumblr of a game site that reposted it; even though this is where I first saw the game, I went back to the original ArmorGames posting because the site's interface is about five times friendlier.