1. Power ups
  * a chance of having power ups spawn on an random unoccupied square when a unit dies
    * +health, +attack, +speed, +jump, (pawn only) remove "attack_only", spawn a new (random?) piece, change movement type
  * then the enemy respawns units, their new units are stronger
  * support for pieces with multiple hit points
  * unit info panel
2. Visual polish
  * clean up tile borders
  * multiple square tiles to add variation
  * make the help text indicate (flash, etc.)
3. Sound effects
  * on hit
  * on piece lost
  * on piece kill
  * on opponent victory
  * if possible, a small bit of background music
4. Further visual polish
5. New units