Took me a while to figure out how to play, but by my fifth or seventh time I got a score of 200.
For folks who need some pointers:
Hearts are used for either Attack or Dig
Diamonds are used for Currency
Spades are used for Dig
and Swords (Clubs) are used for Attack
You get different amounts of reward for stacking certain items in your inventory.
3 Spades give 3 Dig resource, but 3 Diamonds give 5 Currency.
Your final score is based on how many total Hearts you have at the end.
Ah-ha! I figured out a trick to getting a higher score.
The "spreading" enemy doubles itself every 3 turns, and if you kill one part of it, it drops a random piece. If you wait until the late game and set one loose, you can get more drops so long as your Swords and Spades are sufficient to clean up the ensuing mess.