If Sid Meier's Alpha Centauri is the heroin of gameplay, the SMAC(k) if you will, then tower defense is the crystal meth. How appropriate then that some of the best games in the genre involve crystalline gems whose interrelated properties weave together in pyramidal symmetry just like the molecular structure of a jewel.
Gem TD has been out for a while; Gemcraft is just released. They both gripped me with euphoric insomnia, coupled with vitamin C depletion, grinding of teeth, and paranoid hallucinations of creeps, endlessly marching. Like all tower defense games, they evoke a superb linearity of thought, a simplified psychology of marching enemies, marching resources, marching upgrades, ever forward, ever higher. I can't belabor this meth analogy enough.
