Each player starts spread out across the map and gets a hero and 5 "Outlaws" (musketeers) at the start. The whole map is covered in respawning units such as bow men, horsemen, and many others. Each kill gives you 10 gold. Throughout the map there are towns that have buildings (hospitals, towers, gates, wonders) that you can buy with gold. Also in the town are units you can buy (outlaws, arquebus, caribeeners, etc).
So the basic idea is get kills, buy more stuff, and fight over land with the others players (it is diplo).