And one last note, gathering resources automatically like this feels very uneventful. You could let the player gather the drops by clicking on them, Plants vs. Zombies style. As there's not much to do for the player during the wave itself, this would at least keep them busy during the show. :)
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We thought about requiring clicking to collect, but didn't get to it. If we took this one further, in addition to increasing the difficulty, we would probably add click to collect, and a tower that could be placed to auto-collect. We want to balance the difficulty so that you feel like you need to be placing or upgrading towers during the wave.
Thanks for all your thought on this.