How would I go about including code from another .cpp file into the one I'm working on?

Example: An RPG file called game.cpp has all the actual coding to control your character and the levels. A separate file called enemies.cpp has the information with all the monsters in the game. Another file called battle.cpp has the coding for the battle stage when you encounter the monster.

I want to include the other .cpp files almost like functions to be more orderly and to make things easier to edit. The simple #include enemis.cpp wouldn't work, would it, because there would be two main() functions?

Help is greatly appreciated.