Gem TD I encountered first as a map for warcraft 3.  At first, I didn't know what to do, but after playing obsessivly for a few months, I would host games and obliterate all comers.

The game combines 2 mechanics, with a little interaction: placing walls to affect the pathing of incoming monsters, and combining towers into bigger and stronger ones.  The interaction is that the towers are the walls.

There really isn't that much going on, it's just perfectly balanced.  it's just random enough that even if you play perfect, you still lose sometimes.  I don't know what else to say.