[warning: some achievements in this game are currently buggy/unreliable - logging out and back in again may force the achievement to unlock]
For this achievement, you need to upgrade your Town Centre to level 3 on the second day, as this level gives you free walls.
Make sure to upgrade your Town Centre whenever you can. You will notice the empty coin slots that appear when you pass by it. You will need a total of 19 pieces of gold to update your Town Centre to Level 3.
- Level 1 costs 3 Gold (“stand here to light a fire”)
- Level 2 costs 7 Gold
- Level 3 costs 9 Gold
Explore the left and right side of the island to find chests with gold in them, there should be a minimum of three with plenty of gold to pay for the necessary upgrades. Do not bother building walls yourself, as your archers should be able to handle the Greed without fortifications during the first night(s) or you can leave a few coins outside your base to lure them away. Building the one wall as part of the ghost tutorial when starting a game does not void the achievement.
Wait until the morning of Day 2 (when the II appears on the screen) to upgrade to Level 3.* When your Town Centre is Level 3, the two inner walls are automatically built for free, unlocking the achievement.
* Thanks to WinkyWookiee for confirming the upgrade to Level 3 cannot be done before Day 2, otherwise it will void the achievement.