Very cool! It took me a while to figure out what I was trying to do, but once I did, I enjoyed this a lot. The tight time limit made it seem pointless to build the harvesters for the most part (because they take mana, which you could have spent on gathering manually, and a bunch of the resources you're trying to get by building it), though if they were built early on it might have been more useful? (But e.g. the gem harvester would probably not be very useful ever, given how much you need to get before you can build it.) My only successful attempt had me ignore the harvesters entirely.
Anyway, great job with this.