Last time, I talked about features accessible via one spell, such as a magic password that can be read with Read Magic. Another option is to create two linked features, so that two different spells can be used to provide extra benefit. For example, consider the submerged passageway with the dark side passage most characters will miss, but which is easy to see when a Light spell is cast into the passageway. You could provide a second clue elsewhere, such as a crude map etched into a wall with an unreadable inscription. Read Languages will reveal details about the side passage and hints about what is in the secret room. Thus, either Read Languages or Light will "unlock" the additional benefits of the secret room, although other methods, such as wasting a lot of time feeling along the walls of the submerged passage, can also help.
A more elaborate and interesting option is to contrast two or three features. Consider a 1st level dungeon, meant for 1st and 2nd level characters. A 1st level M-U can only prepare one spell, and will probably prepare Sleep or Charm Person; a 2nd level M-U gets two spells. There are a lot of possible spell combos, but limited resources. If you put three different bonuses into the dungeon, each one potentially activated with one spell, you create a situation where different preparations can produce different experiences.
Example: We'll keep the submerged tunnel with the dark side passage as one special feature in our dungeon and add two more:
- An incomprehensible inscription which tells of a special treasure in a room behind a secret door in one of several pits in the dungeon;
- An intelligent (but mute) heavy bronze door with a brass human-shaped head which resists physical efforts to open it.
It may not be obvious, but you could actually use random rolls to make some design decisions here. Draw your map and place several secret shortcuts, rooms, and treasure caches; for each secret area, roll for spell level using the same roll you would for wandering monster level (table on page 10 of U&WA.) Thus, on the 1st dungeon level, there's a 1 in 3 chance the feature can be accessed with a 1st level spell, a 1 in 3 chance it can be accessed with a 2nd level spell, and a 1 in 6 chance each for a 3rd or 4th level spell.
Roll on the spell list for the indicated level, instead of the generic monster level charts. Once a spell is selected, such as Phantasmal Forces, think of a situation that might be solvable with that spell. For example, there may be a sentry point with ogre guards who will only allow other ogres to pass. The M-U could lie on a stretcher and cast Phantasmal Forces to create the illusion of an ogre forcing the adventures to march through the passage, perhaps tricking the guard. To be fair, this particular passage should be a shortcut, rather than a mandatory access route.