As I said before I don't like the "Quit" to continue thing. I'm also not a fan of serializing so the kiddies can pass around solutions.
Additionally the color\symbol on the atoms are too small and nowhere near distinct enough.
These are nits though and otherwise a very thoughtful and great game.
NEEDS a real save feature. "Quit" should not advance to next level either. I'd think a game of this level should be able to handle the common game states and operations without re-inventing the wheel.. and while serializing saves so the kiddies can pass them back and forth might be neat it also trivializes the game to copy\past.
Basic strategy: Turn on autoshot and max arrows, upgrade mana as soon as needed to get to next arrow upgrade. Before you have many arrow upgrade options you will need to re-adjust aim quite a bit. Later w/ele arrows you can leave in one spot most of time. DO NOT ATTACK.. max mana and then get all upgrades you have.. max mana optimally, do it as soon as you can. If you attack and damage castle a large force is sent against you.. if a hard level it will beat your forces back.. let it get into arrow range and then use specials while sending another force to wipe them out. Titan units are key against bosses.. elf is good for last level.