I unlocked this achievement while playing Skull Island. Altogether, there were 22 boulders on Skull Island upon which I could build a tower. When I got to the 5th island during a campaign, it only spawned with 19. At the very least, Skull Island doesn't require a full 5-island campaign to get to it; just load it up in the challenge maps.
I might also recommend playing Skull Island to unlock this achievement, as you don't have access to any hermits that could be used to upgrade to a specialty tower (which nullifies progress towards this achievement). You must build the final four-archer tower as seen in the achievement icon to unlock this.
Granted, you would need to take out all the portals to safely get to all the boulders, but if you have some experience with the base game, you should do okay on this challenge mode. Some suggestions for success:
- Expand as much as possible in the early days and get a sheltered farm within your walls to make easy money. Depending on spawn points for vagrant camps and the Hourglass Statue, I might recommend expanding to one side or the other first; I chose whatever side the Hourglass Statue was on, just to secure it from the Greed as quickly as possible.
- Sock tons of coins into the Hourglass Statue to avoid an instant game over from the volcano in the background. You can keep the volcano at bay for 12 days at a time, and can continue to throw money into the Hourglass Statue every couple of days thereafter.
- I only started chopping down trees next to vagrant camps in order to access boulders once I had already cleared one side of the map of Greed. Leave as many vagrant camps operational as possible (by leaving a tree standing on both sides of the camp) and you'll save yourself a lot of grief and money. The homesteads you can build in place of the vagrant camps suck.
- You will not have access to the Archer, Builder, Farmer, or Swordsman statues for Skull Island, so you'll need to compensate with tons of archers and pikemen. Seems obvious, but worth mentioning since your archers won't be as effective throughout the challenge.
- You do have access to iron right off the bat, which allows you to upgrade the kingdom's innermost walls to max pretty quickly by upgrading the campsite. In an effort to save money, don't upgrade the innermost towers closest to the campfire until they've been automatically upgraded along with the iron keep.