Pitched Battle Achievement
Successfully defend Cuzco without building any new walls or gates in the second Pachacuti mission.Quite simple, just avoid building any walls or gates either stone or palisade throughout the mission.
After the initial battle you will gain a base and your ally will begin building a wonder. Even though you can't build walls you can build castles to defend it. You can also build a wall of houses to slow enemy advances. Whilst the enemy shouldn't threaten you until the wonder is built make sure you have built your defences before then. If you build up a sufficient army you could even go on the attack against the enemy bases to wipe out their forces before they send them to you and prevent them from building more.