TIPS BASED ON AN HOUR OF GAMEPLAY:
The green blobs are mana. Hold down the left mouse button to collect them and get upgrades.
The goal is to keep the humans from destroying the shack. They can damage it EVEN IF THEY'RE NOT ADJACENT TO IT! They can damage it WHILE YOU'RE CHOOSING UPGRADES! They can damage it when HIDING BEHIND THE SHACK! There are no transparent human outlines to show they're behind the shack, so keep clicking everywhere!