Like chess, simple rules, complex play ... only bigger game maps
It's nice to see games like this again, where they have simple rules to start, and you quickly learn various strategies. And just when you think you have a foolproof strategy, it does not work for the next level. Some levels have you placing towers in the alien's path, while others will have you blocking paths while placements are sparse.
After the first few levels, you'll quickly find yourself restarting levels, trying a new strategy. You have limited resources, which grow, and placement and type can be everything. You're on to lasers and canons, but then back to guns and meteors. Flamers work well in some areas, and seem useless in others. And the distortion fields can be the ultimate in a canyon of other weapons.
Definitely an instant classic. So worth the $2 I paid for it on Steam, although it's only $5 normally. Additional packs with more levels and other goodies are $0.99/each.