Hills and Rivers Remain is the most addictive (for me) of the three games I downloaded. This one has a bit of a plot, and some characters, and although these are somewhat thin, there's more to keep your interest than in Crystal Defenders. The idea of the game is that you're fighting a war. Each level has a new map with forts on it, which you can only reach by going down pre-set paths. You start off with a certain number of soldiers, and each 'turn' you will gain more at your base -- the more land you've conquered, the more soldiers you receive. To conquer land, you have to select a number of soldiers to send, and then tap on the fort you wish to attack. The number displayed at the fort is the number of soldiers already there, and the colour denotes what army they belong to.
There are some special 'forts', with additional properties: stables and cannons, for a quick example. Stables allow your troops to move more quickly between forts, and cannons allow you to bombard enemy forts.
At the start of the game you face one other army, but over the course of the game, things get more complex and you face several different armies all at the same time. This can be insanely hard in some levels, but I found out (slowly) that if you get defeated in a level (which happens when all your soldiers are dead) then you can set a handicap and begin the level again, with a higher number of soldiers than everyone else.
You can't change the difficulty, as far as I can tell, until you've been defeated at a level once, which is slightly irritating. The controls can get a little fiddly, too, which wastes a bit of time when you're trying to quickly send troops somewhere.
I'm enjoying it a lot, in any case. I'm not sure how long the game is -- I'm on stage ten at the moment -- but it looks like there's a mode for just playing battles and not going through the gameplay. I haven't really explored that yet. Plot-wise, like I said, it's a bit thin -- it consists of a bit of dialogue between battles, really -- but I wasn't really expecting a deep plot.