to me, that would be the same as normal puzzle games: you have to solve the, er, puzzle. i remember creating a map where the objective was to open a specific door. to do that, you had to place units on specific locations that was spread through the dungeon. however, these locations were just normal regions, they couldn't be seen, so you had to find a collection of amulets that dropped randomly from mobs. these amulets each contained a riddle in their tooltip, solving that would reveal the location of one of the opening regions.
that could be an example.