Iron Curtain Achievement in Assassin's Creed: Revelations
Perform a perfect den defense without using the cannon.
How to unlock Iron Curtain
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 . 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.
Here's a video for further help: