This game, as far as I can tell, is one of the best games on this entire site. Both the difficulty settings and the gem variety add very unique and creative styles to gameplay, and the challenge modifications are just icing on the cake. This one's for sure 5/5 and fav'd - and probably the sole reason I bought a few Kreds. ;-)
TLDR: This game is wonderful.
 "GemCraft Labyrinth"){kind=icon}
 "Kingdom Rush"){kind=icon}