When I was about 16 years old, I really enjoyed playing Travian, which is a real-time browser-based MMO game that allowed you to build and conquer empires. There were however some things that I did not like about that game. Since I was learning to program at that time using PHP, I realized that I could just make my own variant of the game, which is exactly what I did.
World Command was a game in which you could create cities and empires. You could add building and upgrade them. Each building had a specific goal. For example, some allowed you to collect resources and upgrading those allowed you to collect resources faster. Other buildings allowed to you to do upgrades or trade resources. There even was a combat system in which you could train and upgrade armies that could be used to plunder or conquer cities from other players.