It took me a while to figure out what to do here, but then I got going.
This is RTS distilled to its escence... A minimalistic approach, yet successful! I like the way the time mechanics involve not letting the aliens through to the past... gotta keep on edge.
Some thoughts:
-It's hard to hit those arrows in a pinch, they're too small.
- If all your workers get killed you're screwed since you can't get the resources to make more... I ran out of wood and workers so had to just sit there awaiting my inevitable doom. Maybe add some random worker spawn?
Great entry!