Many thanks for your feedback! Yes, level 12 is a hidden Level. SPOILER ahead:
You need to find the correct shape in the background of each level. You will know what to search for if you have a look at the level menu (looking at the circles surrounding the different numbers). The circles differ for the levels. If you completed a level, you can also closely look at the 'O' of COMPLETE when a level is completed, this is the shape you will need to click in the next level :D
