TRIBAL ALLIANCES AND WARFARE: You might allow player characters to somehow become aware that there is a constant fighting going on between the goblins and hobgoblins on one side and the orcs, sometimes with gnoll allies, on the other - with the kobolds hoping to be forgotten
by all, and the bugbears picking off any stragglers who happen by. With this knowledge, they might be able to set tribes to fighting one another, and then the adventurers can take advantage of the weakened state of the feuding humanoids. Be careful to handle this whole thing properly; it is a device you may use to aid players who are few in number but with a high level of playing skill. It will make it too easy if there are many players, or if players do not actually use wits instead of force when the opportunity presents itself.
MONSTERS LEARNING FROM EXPERIENCE: Allow intelligent monsters (even those with only low intelligence) to learn from experience. If player characters use flaming oil against them, allow the monsters to use oil as soon as they can find some. If adventurers are always sneaking up on them, have the monsters set warning devices to alert them of intruders. If characters run from overwhelming numbers, have the monsters set up a ruse by causing a few to shout and make noise as if there were many coming, thus hopefully frightening off the intruders. This method of handling monsters is basic to becoming a good DM. Apply the principle wherever and whenever you have reason.
EMPTIED AREAS: When monsters are cleared out of an area, the place will be deserted for 1-4 weeks. If no further intrusion is made into the area, however, the surviving former inhabitants will return or else some other monster will move in. For instance, a thou1 might move into the minotaur’s cave complex (I.), bringing with him whatever treasure he has.