In this video, you'll learn about the basics of using your military to defend your fortress, in this case against a dinosaur attack, in Dwarf Fortress.
Slaves of Arnok II: Dwarf Fortress, or Dwarf Fortress as it is more commonly known, is one of the most intimidating, challenging, and rewarding PC games in the world. It is totally free to download, available for Windows, Mac, or Linux, and would probably run on a 386. It's a text-based strategy RPG on an epic scale, where you create an empire starting with only seven dwarves and some paltry supplies.
If you're having trouble with it, you're not alone. It's really hard. Click here to watch other how-to videos covering many aspects of the game.