Perform a perfect den defense without using the cannon. (20 G)

The first time you experience a Den Defense is in Sequence 2 Memory 6. You are on the roofs and must place your units and barriers around to defend. You can use everything except for the cannon, which is used with LB. When the game tells you about it, just ignore it. Put barriers whenever you can. The last wave of enemy is a ram unit. The den must take no damage. You will unlock the achievement in the stats screen afterwards.