This one's excellent! I found myself making constant decisions as to where I should focus my attention, attempting to ensure a stable balance between the collection, defense, and objective completion.
The idea of one pool of resources for all actions means that there's always a tradeoff for committing to one pillar of the game loop. The dwindling resources across the map additionally create a natural ticking clock as the difficulty steadily increases.
I hesitate to give suggestions here, as there's already a very well-defined balance in the existing mechanics. More enemy types with additional house upgrades might be a nice thing to explore?
Excellent work!