Nice game, I like the combination of jigsaw puzzle and tower defence. I agree that it feels like it has potential for more strategic depth - as it got faster I found I was often just hurriedly plugging gaps with individual pieces rather than paying attention to joining them up. Perhaps more opportunity to plan in advance would improve that, and (as you've said in other comments) some abilities that could be a reward for joining pieces in certain ways. But it's a cool and fun idea, well implemented in the short time window.