It is a simple enough matter for the GM to arrange for the PCs to encounter new, more powerful foes, but it is often more desirable (although often more difficult) to introduce instead complexities in the task of overwhelming the old ones. Problems that have nothing to do with fighting or overcoming enemy creatures also need to be considered and placed into the campaign. These may take the form of labyrinths and mazes to be negotiated, mysteries to be solved, information or objects to be gathered, riddles to be interpreted, and who knows what else.

Gary Gygax
Role-Playing Mastery 49-50