Designing enemies is fun because it can be very interesting with the game you are playing. In games where you can free roam like Zelda: Breath of the Wild and Skyrim, you will encounter a plethora of different enemy types. With enough experience you could add enemies from other games or create your own. This allows you to add any type of enemy you can think of and face it in any which way that you want to, however, you need to make sure that the enemy type you are creating works well with the environment you set it/them up in. For example, putting a giant inside of a cramped space would be ill advised, find the type of area that works best with the enemy you want to create and use that in order to insure things turn out they way you want them too.
