Solid tower defense game. I think it's quite the bargain for its price... and I don't even know how - or when - I got it!

The quest line is simple. Progression is seemingly indefinite and constant.

The gameplay is simple, though if you want to be a sweaty min-maxing wizard, you'll definitely want to use a third party app (wGemCombiner, or Gemsmith) to simultaneously make the best quality gems... and save time making them.

Gemcraft - Chasing shadows, was made using Flash; once you start dipping your toes into Haunting difficulty (Hardest difficulty), there will be thousands of reavers, swarmings and/or giants for the game to render... and your Frames Per Second will turn into Seconds Per Frame.

Oh and your mana will hit the exponent number range well above 1e9+... most of my runs end up with my wizard having more mana than he does atoms in his body.

Again, you'll want to run a third-party tool. But this time it is to ease performance for the game:
https://github.com/ZS-NVB/GCCS-Performance-Patch

Legendary game from the good ol days of Kongregate

The most complete tower defense game I've ever played. Whole-heartedly recommend, it has everything you could ever want. XP, levels, skills, equipment, difficulties, modifiers, infinite leveling, mulltiple different ways of playing, etc..