Iron Curtain

This achievement can be easily obtained during the Den Defense tutorial in Sequence 2, Memory 6 - On the Defense.? Go back and play it and don't use the cannon salvo or cannon shot.? The tip telling you to use the cannon will stay up for quite some time, but you are not required to use it to finish the memory.

1. It is easiest to do on this level, because you keep all of the units you've unlocked, plus all of the enemies are very easy.
2. Make sure the last barricade does not get destroyed or it will count as a fail