0.A.D.
When it comes to free real time strategy games, 0AD by Wildfire Games is one of those that stand out. Although under heavy development, the game is already very playable. Like other RTS games, you have to manage resources, build an army and manage an economy.
There are six civilizations: the Carthaginians, Romans, Celts, Hellenes, Persians and Iberians. Each one has strengths and weaknesses. When you play the game, the resources are displayed at the screen bottom. The mini-map is at the left. To select all units nearby, double click. You can assign units to perform tasks like building structures. Of course you can order them to attack the enemy.