Of course, villains are the main dish. But what would be a game without the common enemies the player has to heroically defeat one after the other, making his way guns blazing or sword swinging?
Well actually, Shadow of the Colossus is a fantastic example of a game where there are no common enemies, but only bosses. But that's an exception and is part of what makes it an extraordinary game.
Generally speaking, a game with enemies needs common enemies. Creating enemies is one of the design activities I enjoy the most. Inspiration can come from anywhere, depending on what kind of game you're doing. Enemies can be soldiers from a hostile faction, monsters, aliens, robots, cannibal plants...the only limit is your imagination! ...
