In order to unlock this achievement, you need to upgrade the archer tower (the first fortification), to level 5. The tower costs 200 wood initially, and takes 5 minutes to build. Your keep will need to be in the 5th Age before you can upgrade your Archer Tower to the 5th level. The subsequent levels and cost/build time are as follows.
Level 2: Costs 1,600 wood and takes 30 minutes to build.
Level 3: Costs 2,750 wood and takes 2 hours to build.
Level 4: Costs 4,500 wood and takes 6 hours to build.
Level 5: Costs 14,500 wood and takes 16 hours to build.
I hope this solution helped! Leave a comment letting me know if there is any room for improvement! Thanks.