Rampant Games - RPG Design: Hard or Soft Progression?
Jay Barnson has penned an editorial on progression in games and recognizes four basic mechanisms to accomplish progression, listed in order of "hardness" as he sees a trend towards games becoming more linear and plot-driven.
#3 – Obstacle-based: A part of the game is blocked by a barrier that cannot be removed by brute force, but rather by a not (strictly) story-driven mechanism, such as finding a magical key, learning a mantra / password, or obtaining a means of travel that allows passage. It does allow the player to progress at their own pace, while forcing them to explore other areas to obtain whatever is needed to clear the obstacle. These can also serve as a reward in and of themselves, as it’s kinda fun when a formerly impassible barrier suddenly ceases to be an obstacle. However, frustration can rise when it is unclear how to overcome the obstacle, or when the obstacle fails to make sense in the logic of the game – for example, when characters can destroy dragons with ease, but a simple wooden door acts as an impenetrable barrier.
Examples: The Silver Serpent in Ultima III blocked passage to the island of Exodus unless you used the magic word, “Evocare.” And about half of any Dungeon Master-style game, including the recent indie release Legend of Grimrock, where puzzles and locks make up much of the gameplay.