There are 3 achievements unlocked through the main game. To get started, boot up a game on any difficulty and back-out. You'll then need to click 'themes' and click a different theme to what you had on when you booted up the game. Now start a standard game on hard mode (or any other difficulty), which will unlock this;
Now as you've noticed there are a lot of tiles, you'll need to match up the same ones to each other and they'll uncover the ones underneath or connected to them. If you get stuck you can press 'H' on the keyboard for hints and doing this can take up to one hour to complete, thus netting you the hard and complete achievements;
Congrats on your 100%!
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