Your capital city is the permanent heart of your empire. You get new items for your capital city by playing classic Triple Town, and as your Capital City grows, it generates great rewards for you!
Unlike in a normal Triple Town game, which has a single possible build path (grass -> bush -> tree -> etc) there are three possible build paths in the Capital City: farming, industrial, and military. Each of those build paths terminate in an ultimate building, and each of them generates different types of rewards for you over time. You can freely earn all three of the Capital City item types simply by creating treasure chests or matching 4+ items in any normal game of Triple Town.