To access the level to complete this achievement you will need to unlock and complete the purple vortex levels.
You will also need to active four heads that appear on Year 25, 45, 68, and 77. You will know they are active because their eyes will be blue. If you are going back to these levels then you only have to active the eyes you don't have to finish the level.
Once you have done that, on the level select screen on the last page there should be a big purple button.
If you press the purple button you will be taken to ???. Once on the level you should see five heads in the ground, one of which will not have blue eyes, simply place the tower on the blue crystal next to this head and the achievement should pop.