The way a 0 A.D. game develops is exactly like that of Age of Empires II. You'll start with a few villagers and scouts. Your goal is to build a big city full of buildings, walls, and other defensive elements; this will be your capital from which you can destroy your opponents or make alliances.
To build buildings, improvements, and units, you'll need resources. Meat, stone, wood, and metal are the main resources, and to make sure you always have some, you'll have to keep your villagers working around the clock.
In 0 A.D., there are nine different civilizations to choose from, including the Greeks, Romans, Persians, and Britons. Each of these civilizations has its own buildings, strengths, and units. All the details have been customized to represent the historical reality of each civilization as much as possible.
Although 0 A.D. began as a mod for Age of Empires II: The Age of Kings, it soon became a game in its own right with its own graphics engine. And it shows. 0 A.D. is more or less as visually appealing as the third installment of the Age of Empires franchise.
0 A.D. is an outstanding open source and real-time strategy game. Best of all, it's free. Despite being in the very early stages of development, it offers a better gaming experience than many similar but more commercial games.