Many thanks for your detailed feedback! You are right regarding level difficulty. I thought it would be nice to 'mix' the levels instead of increasing difficulty all the time. This way sometimes player tend to think "too much" instead of going for the simple approach, do you know what I mean? :D
And yes, Level 12 is an easter egg!
So SPOILERS ahead:
You need to find the correct circular shape for each level, click it within the level and complete the level as well. The vorrect circle is shown in the level selection screen or the 'O' of 'COMPLETE' when using the "next-level-menu-approach" :)