There's only one tower and enemy type, so your score boils down to: 1) how well you can utilise powerful environmental effects when placing towers; 2) how complex a maze you build for the enemies; and 3) luck of the draw - how perfect your level generation was.
Levels are randomly generated as per a seed (except for tutorials/player created levels), so the fun comes in trying to find and master the elusive "perfect level". You are encouraged to share or borrow seeds from your friends/community, but the thrill is still present even when hunting alone.
For more crafty individuals, you can create and share your own levels simply by copying/editing/pasting JSON (instructions in-game, under Advanced > Copy Example JSON). Keen to see your creations!