This achievement requires that you purchase all parcels of land to expand your kingdom to the maximum size. You start with one plot of land, but must purchase the 79 other plots. The price of land increases by 5,000 gold each time, until reaches its maximum price of 100,000, for which it remains constant. Purchasing all plots demands around 7 million gold. This will require some serious gold accumulation. See the "General Tips" section in the Roadmap for the best method to grind gold. You should wait until you unlock the windmill as it will be your cash cow. This will likely be your last achievement.