Also/instead, once the dungeon inhabitants come to realize that adventurers almost always follow into and out of the dungeon, the more intelligent ones will begin to avoid that area, making their lairs in the more remote and defensible areas (since they generally don't care about getting to the surface, what serves as an obstacle for the PCs might not necessarily be to them), meaning that if the PCs stick to the "safe" areas they'll find nothing but empty rooms filled with debris, vermin, wandering monsters, fellow surface-dwellers (bandits, NPC adventuring parties), and perhaps some poor/weak monsters that were thrust out of their original lair and have no place else to go (which can be either a boring and unrewarding combat or an interesting role-playing encounter -- perhaps these monsters are looking for revenge and might be willing to team up with the PCs to achieve it, or perhaps they'll pretend that's the case but really they're wanting to deliver the PCs to the boss-monsters as an offering, to prove their value and loyalty). In order to fight the toughest monsters (and even moreso to get the best treasures) the PCs are going to eventually have to depart from the "safe" paths and start doing some real exploration.
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