Bsrgvty

The other answers seem to think that the PCs have the Goblins trapped. The map tells another story.

By camping out at the entrance to area 6, the Goblins have the PCs trapped.


The Goblins have access to water and food and fresh air. The PCs don't.


Goblins carry a bunch of wood to the two choke points, maybe on a wagon, then break the wagon, set it on fire, and back off to where there is fresh air holding ranged weapons.


The PCs may hear some activity down the tunnels. If they don't investigate, the next thing they get is smoke flowing through the tunnel, and their air going foul. They now have to fight their way through a burning fire trap into a goblin ambush, with filth-covered spikes blocking forward progress while goblins riddle them with arrows.


The bridge at area 5 is rigged to collapse. To the west of 5 there are filth-covered caltrops. A large fire inside the tunnel at area 4, then more filth-covered caltrops and then the river.


The fire at area 5 will take longer to set up, and the goblins may screw up by setting the fire at area 4 too early.


Goblins love the upper hand, and your PCs just gave it to them.


Alternatively, the Goblins simply flee with their treasure. The PCs already killed about half of them.

Don't just "handle" them; find a way to guide an enjoyable experience.

As an example: Hold a strategic meeting for the Goblins that become aware of the situation, let the players listen in. Support their roleplay and adjust.

The MM on Goblins, page 165-166 offers common strategies and ways to play Goblins:

They crave power and regularly abuse whatever
authority they obtain. [...]
Goblins are lazy and undisciplined, making them poor servants,
laborers, and guards. [...]
Motivated by greed and malice,
goblins can't help but celebrate the few times they have
the upper hand. [...] Goblins are ruled by the
strongest or smartest among them.[...] Goblins festoon their lairs with
alarms designed to signal the arrival of intruders. Those
lairs are also riddled with narrow tunnels and bolt-holes
that human-sized creatures can't navigate, but which
goblins can crawl through with ease, allowing them to
flee or to circle around and surprise their enemies.

Also, look at the stat block for strategies employed:

Nimble Escape. The goblin can take the Disengage or Hide action as a bonus action on each of its turns.

If they see no way of winning against your ambush they might try to get their family involved, which may end badly for them:

Goblins belong to a family of creatures
called goblinoids. Their larger cousins, hobgoblins and
bugbears, like to bully goblins into submission.

There are also more general options to move the game along explored in the DMG.

This thread explores player agency when you want to move the game.

If you let them wait out the encounter and the Goblins starve to death, as suggested in SeriousBri's answer and you let the six weeks passage of time severely impact the campaign, then not awarding experience points is inconsequential reasoning.
They won, and they paid their price. They solved the encounter by routing/starving unless the state of the encounter changed they have successfully dealt with the encounter.

DMG Experience Points, page 260:

When adventurers defeat one or more monsters-typically by killing, routing, or capturing them-they divide the total XP value of the monsters evenly among themselves. If the party received substantial assistance from one or more NPCs, count those NPCs as party members when dividing up the XP.

The Question is: is the encounter solved, the threat negated, the risk avoided?

MM Experience Points, page 9:

the DM may also award XP for neutralizing the threat posed by the monster in some other manner.

PHB Preface on enjoying the game, page 4:

Playing games with your friends is a lot of fun, but D&D does something
more than entertain.
Playing D&D is an exercise in collaborative creation.
You and your friends create epic stories filled with tension
and memorable drama.

Think about the consequences of the player's actions, read the mood of your table. Punishing your players for coming up with creative solutions reinforces that creative solutions are not wanted at your table and may turn your collaborative storytelling into apathetic players who react instead of being proactive.

I am not fond of methods to punish players for unexpected conduct. I say, embrace the situation.

First, as another answer said, laying siege to a group of goblins for weeks and weeks has other consequences. On the other hand, your party now has a reputation as a band of bloody-minded people who are willing to find workarounds for a tough situation.

Secondly, if you were a goblin, what would you do?

• Try to escape


• Make your own best group attack using whatever ability you had
  working behind you (might punch through, might overpower the enemy’s superior position).


• Try to negotiate with the big people who are laying siege to them.


• Try to deceptively negotiate.


• Etc.

Having said all that, it is also both possible and fair for a group of players, with their multiple real human brains, to come up with a viable fast workaround for some scenarios that you give them.

It is okay. This happens. It means they’re dedicated to the game, probably. Just congratulate their lateral thinking and move into the next challenge. If you think there’s another situation where they will use the exact same tactics, maybe you will have a workaround prepared. After all, the same approach doesn’t always succeed.

And if they don’t use it for a while, then come upon a really bad situation later in the campaign, then look at each other and say “remember that goblin cave we were in?” And they all nod at each other and set up an ambush, even though the odds are worse and they’re way different people, maybe it’s good for their tactic to succeed again, or partially succeed. That would be narrative symmetry and everyone would probably remember it as a good scene.

The thing is, you never know what the other person is going to do. Ever watch “a perfect murder” with Michael Douglas? If your plan is dependent on the other person’s actions, then prepare to be thwarted. This doesn’t make the party bad or trying to play unfairly. It makes them real, thinking humans.

I mean, if I were literally, in real life, in their situation, I would try everything I could think of to gain a decisive advantage with as little risk as possible. Wouldn’t you?

In my opinion, some of the other answers here are needlessly harsh, especially if newer players are involved. And there’s not much about how to use the Last Mine of Phandelver module, which offers useful guidance.

While some of this advice is “after the fact” it should provide guidance on how to deal with similar situations in the future.

Giving the party credit

Your party’s plans is not half bad at face value. They rescued the hostage, and are trying to get the drop on their enemies. This plan will let Sildar stay safe in the back of the upper level in that room. So they’re thinking, that’s good.

You let them pull off one ambush, and then got the notion that it was time to move on. That was a good call on your part.

When the party wanted to “rinse and repeat” the ambush plan, they were pushing their luck. Do they really expect the goblins to be that dumb that they come into the room by one’s and two’s until they are all dead?

(Your player with DM experience very well may have been thinking about what he would have cooked up for the party in that situation, and reckoned it was better to move on.)

Violating Exploration as a Pillar of Play

The game is simply most fun when exploration is part of it.

In your case, your party busted into a room and launched a successful attack, then stayed there to “defend” it. That’s about all that room has to offer.

Fighting in the same place over and over is going to start to get repetitive (boring, for some) even if some of the players get a kick out of making the place a “goblin grinder.”

Exploring new areas and having battles in them is part of what makes the game interesting, so definitely nudge the party in that direction. But you still want to find the fun as you do this. That is, don’t murder the party or otherwise punish them too badly.

You may want to mention this to the players, out of session.

Know your (party’s) enemies

When you’re not sure how monsters in a dungeon would react to some unexpected action of the party, you may want to cogitate on the module a little more. Get inside the monsters’ heads. When you do, you can make the adventure come alive more, even when the party plays it straight.

The Cragmaw Hideout

We know a little about the other denizens in this dungeon:

The “dungeon boss,” Klarg, “is filled with delusions of grandeur and views himself as a mighty warlord just beginning his career of conquest. He is not entirely sane… The goblins under his command resent his bullying.”
Then again, Klarg is wiley, and will “hide behind stalagmites while the goblins take cover behind the piles of supplies, hoping to ambush the characters when they enter the cave.”

(LMOP, p. 13)

The goblins in the Twin Pools room built a trap, that they are eager to use. They may devise some plan to draw the party into it.

(LMOP, p. 12)

There are a lot of ways you could go with this information, but note that these are all crafty creatures, not so likely to follow their compatriots into the same trap over and over.

Some more possibilities:

A goblin might guess invaders are present and whisper down the hallway, trying to enlist their help to dispatch the bully, Klarg.
Klarg might shout orders, threats, and insults into the room where the party: “Come out, you filthy maggots, or I’ll crush your heads.” Maybe Klarg knows he’s talking to invaders, maybe he doesn’t.

If you need a moment to think what the monsters might do, take it. Get a drink, or whatever. There’s no need to rush.

Sildar Hallwinter

In your case, you’ve also got a NPC well-suited to guide the party.

Sildar might suggest the goblins are probably onto the ambush plan and are likely to arrive in force if they come again. (Wise players understand not to ignore the prescient warnings of their adjunct NPC’s — somehow, what they predict uncannily comes to pass!)

Some more things specific to the LMOP:

Sildar can also guide the party towards achieving important goals, like reaching Phandalin, finding Gundren Rockseeker and Iarno Albrek, and/or finding the goblin’s headquarters, Cragmaw Castle.

It wouldn’t hurt for Sildar to mention that his organization is willing to offer significant gold to a party willing to help with these goals.
Sildar may also have noticed the treasure chest in Klarg’s chamber, noting the bugbear might just abscond from the dungeon with it.

Wandering Monsters

It’s also easy enough to let the party get this urge to ambush out of their system by throwing wandering monsters at them.

The wandering monster table, Wilderness Encounters in the Triboar Trail section (LMOP, p. 27) could be used — although you may want to nerf the encouters a bit.

There’s no need to roll the d20 to see if somebody shows up — if the party waits for an encounter, just roll the d12 to see who shows up for dinner.

If a player challenges why there would be random monsters in a goblin cave, you can point out how the guards at the door have been dispatched.

Just make sure the monsters are penniless and have no valuable information for the party (disregard the “bounty” note for the hobgoblins — that particular group doesn’t show up until later).

An ogre, for example, might announce to the party that he just wandered in, and has no treasure or useful information, just a club for smacking heads. The message being: go ahead and have some fun fighting, but this tactic isn’t going to advance the plot.

After every fight, Sildar can make an observation like, “I’m not sure what you’re trying to accomplish here. Isn’t it time we moved on?”

Once the PC’s are sufficiently beat up, they will want to get out of there.

Onward in the module

Later dungeons in this module have reasons why the party should avoid this strategy:

The party should be motiviated to seek the death or capture of the wizard Glasstaff, who may slip away if he discovers trouble.

A hunting band of hobgoblins will return behind the party if they take too long.

Wave Echo Cave is inhabited by wandering monsters, who will inflict wear and tear on the party without advancing their aims much.

I would have let the player have their ambush for a few more hours, then the players would hear some scraping and noises from the caves.

If they didn't move, then the noises continue for 3 hours, then stop. If they come out sooner, they find the goblins packing up and abandoning the cave.

If they wait the full three hours or more, the goblins have legged it out an hidden escape tunnel rear of the cave. When they discover this, they get 1/2 to 2/3s the remaining XP and no more loot for the encounter, because they have solved it mostly but the goblins took everything with them and will continue to be a threat in the future.

If they want the rest, they can try to track down the fleeing goblins and and the goblin threat, at which point the get the rest of the XP and all the loot. This could be an interesting side excursion, and the difficulty changes based how long they were willing to sit tight.

The way it says in the rules (PHB p.6)

1. The DM describes the environment


2. The players describe what they want to do


3. The DM narrates the result of the adventurers' actions

Well, 1. and 2. are dealt with so what are "the result of the adventurers' actions"?

It is likely that "the result" will be some sort of reaction on the part of the goblins (and others) inside. It is unlikely that they will choose to starve to death so they will probably do something confrontational. This could range from a series of probing attacks (goblins (and others) are quite Sneaky and may be able to surprise the ambushes) to an all out no-holds-barred assault - which will almost certainly kill all the PCs (and serve them right - don't try an ambush an unknown force which may be superior to yours).