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The castle is your main building and the only one you will start out with and acts as the center for your growing empire. The castle is built on a crystal that supplies power to your entire city. You can upgrade your castle and use technology to expand your empire and gain new cities.
The market in Freesky Online is very important to your success. At the market you can sell your ship parts and extras resources and in exchange receive gold and more importantly experience points. These points can be used to expand your empire with technology. You can also gain these experience points engaging in combat. Using the market is one of the only ways you can get gold for your empire besides combat. Gold is used to upgrade buildings in the game so your economy is vital to your expansion goals.
Another large part of Freesky Online is your technology. You have several technology institutes in the game including military, cultural, and natural. Each building provides different sets of technology for your empire. You have military for your armies, natural for production increases, and cultural for expansion. At these buildings you can use your points to upgrade different technologies.