Go to the credits screen. Then input the commands (you have to do it quickly). Spoiler alert: unlocking all of this with friends is much more fun than using this code. Use responsibly.

Unlockable	How to unlock
Unlock all archers and levels (Controller)	Left - Right - LeftTrigger - RightTrigger - Up - Up - Down - Left - RightTrigger - A
Unlock all archers and levels (Keyboard)	Left - Right - Tab - Shift - Up - Up - Down - Left - Shift - C