Every game (except for experimental and experiential games) needs an objective for the player, something they must strive to do, not just within specific levels, but across the game overall. This objective is important not just for the player (to make the game fun), but also for the developer to decide how challenge, diversity, and interest can be added to the mix. Before starting development, have a clearly stated and identified objective in mind.
Challenges are introduced primarily as obstacles to the objective--and bonuses are things that facilitate the objective--that make it possible and easier to achieve. For Dead Keys, the primary objective is to survive and reach the end of each level. Zombies threaten that objective ...
