I now have a couple projects in the works that use the same dungeon map twice. Earlier in an adventure (or in an earlier adventure), the heroes go through the dungeon. Later, they come back to it, and go through it again. Why on earth might I do such a thing? A few reasons:

* Familiarity. A reused map takes away some of the burdens of exploration (in the parlance of my prior blog post, the heroes jump right to phase two), allowing more focus on the events at hand.

* Encourage In-Game Thinking. The heroes spend so much of their time off their toes and not knowing what to expect that planning around something they expect is something they relish. Reusing a map means the heroes know the configuration of the rooms as they go in, and can use that to their advantage. Note that this is slightly different than the players knowing the whole map configuration--as may be the case when the GM puts the whole map out on the table but tells the players, "your characters only see this part here and this part here." In that latter case, the player must "play dumb" a bit to keep their characters from acting on that knowledge (and some players are better at this than others). In the former case, though, the characters know just as much as the players do, which means the players don't need to hold back any knowledge.

* Allow Players to Demonstrate Mastery. Even if a dungeon is a suite of dangerous deathtraps and flowing lava, the heroes will feel more comfortable there if they know what to expect and are aware of how to overcome its dangers. This makes the players feel like their characters are experts at dungeon-delving, and anything that makes the players feel like their characters are good at something should be included in your games! A good GM can double down on this by making the new inhabitants in the area the second time the heroes visit be less informed than the heroes about the dungeon. This inverts the knowledge issue presented above, requiring the GM to "play dumb" a bit. Maybe the evil necromancer and her undead minions have moved into the magma dragon's lair, but they don't know how to deactivate the wall of lava trap and therefore don't go into the western rooms. If the heroes know about the wall of lava trap, they can use it to access some "safe" rooms and maybe turn the tables on the necromancer who has no idea what's in those chambers--or that they lead to a secret door to her treasure room.

* Maps are Expensive. From a purely monetary perspective, good cartographers are expensive. Reusing a map can save money, and reusing a map well doesn't look quite so much like you're just trying to save a few bucks.

Reusing a map doesn't mean reusing all the encounters. The second time through should have just as many thrilling fights and interesting encounters as the first time through. In fact, it might have even more; the GM might increase both the number and difficulty of the encounters so the heroes spend more time and resources in the area the second time around. Here are some pointers about how to reuse a map well:

* Changes. Consider what realistically might have changed since the heroes were last there, then change all that. Then change even more than that. More changes are better! The more different the encounters are, the more exciting the heroes will find their second foray into the same area. This doesn't just mean thinking about new inhabitants--although the new inhabitants should be different from the others (warrens filled with goblins might have been repopulated with ghouls, for example, or a sewer the heroes cleared of smugglers might have had wererats and giant vermin move in). Think about what the new inhabitants need, and about their ability to make gross changes to the area. Perhaps the new inhabitants erected a wall to close off easy access to an area, or maybe they stacked several wooden tables to make a shooting gallery. 

* Have Callbacks. If the heroes made any physical changes to the environment the first time they were there, keep at least some of those. If the heroes come across a sprung trap they previously triggered, corpses of previously slain foes, or evidence of past battle-damage on the walls, that's neat for them because it shows they're having a lasting impact on the world. Keep the new inhabitants in mind, though; a dungeon repopulated with flesh-eating ghouls isn't likely to have many corpses still laying around. 

* Build in Secrets. Even a single secret passage is a great add to a map you'll reuse--and having more than one is even better. It's nice for the heroes when they find these in their first search. But it's even nicer when they can use them against their enemies in their second foray. Bonus if the new inhabitants don't know where all these passages are, so the heroes can sneak around a bit and surprise them--that makes the players feel levels of terrain mastery they don't often feel when exploring dungeons. But this isn't always the case; the new inhabitants might have found (and might have trapped!) formerly safe secret passages. 

So, with these points in mind, should you reuse a map a third (or fourth, or more) times? Each time is going to get slightly less interesting for the heroes, so the changes should be more extreme. Eventually, the changes need to be extreme enough to keep the heroes' interest that a whole different map might be better. After that point, the key reason to return to the old map becomes nostalgia more than anything else: "Remember when we had such trouble with the kobolds here? And then the hobgoblins?" "Yeah, shame to see this old place overrun with trolls like it is. Let's take that ol' secret passage around to the throne room, cut through the treasure room, and draw the trolls into the fiery forge where we can finish them off for good."