The restart button should restart the level you are at instead of going to the menu. Also, try to add a small margin to the diamonds produced in the level vs the required to complete it. I was 219/220 diamonds in level 4 and it was rather frustrating.
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Same, I stopped there due to that.
Also, it doesn't save?
Well, apparently you can just click any level at any time. Great! And now it's clear why level 4 only has 219/220 required diamonds. That's the end of the demo. There are no more levels.
There's no exit-level button (well, 'Restart' IS that), and if you try any levels past 4, you're stuck and have to refresh the page.
There doesn't seem to be much game here yet.