After looking at dungeon design and how light works in the game, I thought I'd spend some time looking at how Wandering Monsters actually work in the Moldvay edition of Basic D&D (which isn't completely how I *thought* they did).
First we see how often Wandering Monsters appear:
- At the end of every 2 turns, the DM should check for Wandering Monsters. To do so, roll 1d6: a result of 1 indicates that the party will encounter a Wandering Monster on the next turn."
However in some areas you would check more often:
- The dungeon may have certain areas where Wandering Monsters are encountered more often (such as on a roll of 1 or 2).
- The dungeon may have areas where the DM checks for Wandering Monsters every turn, or where a monster will appear when a corridor is entered.
In addition to this Wandering Monster frequency will be affected by the choices the behaviour of the adventuring party:
- Wandering Monsters should appear more often if the party is making a lot of noise or light, but should not be frequent if the party spends a long time in one out-of-the-way place (if they stop in a room for the nigh, for example).
Also important to note is the distance at which a wandering monster is encountered.
- The Wandering Monster will be 20-120 feet away from the party when encountered (roll 2d6, multiply the result 10) in a direction of the DM's choosing, and will be headed toward the player characters.
If the characters are in a 20'x20' room and the monster is 70' away it's reasonable to assume that while it may pass by the door of the room as it goes about it's business, there's no reason to assume it will automatically try to enter the room the party is in! The players choice of what sort of room to rest in will be an important strategic decision.
It's also possible that the party may somehow detect the approach of the monster and find a way of avoiding it, or even scaring it off.
When determining the exact Monster encountered it's worth noting:
- Wandering monsters may be determined at random or selected by the DM.
Examples also mention the possibility of encountering bats or rats (which aren't always dangerous encounters) as well as providing some example tables that include Lawfully aligned Dwarf, Gnome, and Halfling encounters as well as encounters with Acolytes and Traders who could be any Alignment. Certainly every time a Wandering "Monster" appears the intention isn't that there will be a combat encounter.
So there's more to wandering monsters rolling a d6 each turn and starting combat if the result is a 1.
Wandering Monster checks are largely based on where in the dungeon the party is and what they are doing. Player choices affect the frequency of these checks. Even if a Wandering monster appears player choices and strategy can determine whether there will actually be an encounter, and further choices and strategy will determine if that encounter will necessarily lead to combat.
Like Light in the Dungeon I think these rules may not be used completely by a large number of groups, but in omitting them an important strategic element of the game is being removed as well.