From my experience with dwarf fortress, what you want to do is trap the dragon and use it as a source of FUN. Basically trap it and use it to terrorize prisoners and bad dwarfs.

Lava

The funnest way is to create a trap passage way. One roughly double the length of Glori and positioned so that if he wants to get access into the inner sanctum of your city, it is the easiest way through (aka block the other passages, or station them with a ton of guards to deter Glori from entering those ones). Once Glori enters the tunnel completely, you shut of both ends and open the trap holes in the ceiling. Pour some Molten Lava into the room, enough so that Glori will be left with just his head sticking out of the magma. Let it cool down and now take the dragon entrapped in solid stone out of the room and put it somewhere nice and view able. (If its really cold, you could do it with water and ice. But that would only temporarily contain Glori, which is more than enough time to deal with it).

Spike Traps

Another answer straight out of Dwarf Fortress, Spike traps. These are traps in the ground that when triggered cause up to 10 weapons to shoot up out of the ground (I say weapons because it let you throw in anything, but spears or pikes are what you realistically want). Force Glori to fly low by creating low tunnels or placing large metal pikes in the ceiling to force Glori closer to the ground. Once Glori passers over the spikes, he either triggers the spikes or your Dwarves trigger it, impaling his soft underside.

Build a Wall

As we all know, dwarves are master builders. Simple find the area Glori is in and seal it off. No one is as skilled as Dwarves in creating solid walls of rock and once they seal him off they can listen to the angry cries of Glori in peace.

Flood it all

Water is annoying and can be extremely powerful. Tap into an natural aquifer or water source and allow it to pour through your tunnels into the depths. Creating in it sealed off passage way which is sealed off from the main kingdom will allow you to quickly identify potential routes for Glori to get into your fortress. Find some water flowing in a tunnel? Seal it off. Of course, this might make Glori fly up even faster, but dwarven ingenuity is a double sided axe.

More traps

Okay, know these traps are big traps that take a long time to reset. Examples are the swinging blades from Game of Thrones. and The Boulder from Indiana Jones. Once you manage to lure Glori into the traps range, you activate it and pray to God it works. In this case, I suggest massive smithing hammers like those shown in The Hobbit, that the dwarves used to forge equipment. Put a dragon between those and they're dead. Otherwise, a convenient giant statue of a hammer or some massive stalactites should be enough to do the trick.

Standard Approach

There is no victory without sacrifice. Prepare your soldiers with shields and armor. Put spikes on them to deter Glori from biting them and hopefully focus on flames more. Pair your dwarves up, with one throwing bolas, nets, chains or spears (with chains) and the other with a large shield to block the flames. One blocks the flames, the other throws crap at Glori until he is weighed down and can no longer fly. Once he's on the ground, just keep hacking at him.

Note: I assume your dwarves are actually masters of architecture and mining. Sure there might be hundreds of unknown tunnels, but your dwarves should know exactly what they have mined and which passage ways lead to where. No dwarf wants to be the one who accidentally brings down the entire kingdom by mining through a support wall and no dwarf wants to go out searching for gems only to end up at someone else search site or even worse lost. It might be confusing to us humans, but dwarves know their stuff.

Hit it in the voonerables

'Im sure a hero will turn up in time,' said Carrot. With some new sort weapon, or something. And strike at its voonerable spot.'

'What's one of them? Said Nobby

'A spot. Where it's voonerable. My grandad used to tell me stories. Hit a dragon in its voonerables, he said and you've killed it'

Lacking a hero with knowledge of the dragon's voonerables, you need to be a little more systematic about this.

First you need to stop Glori flying. The great advantage of dragons over dwarves is air superiority combined with ranged weaponry. Fill the space where a dragon could fly with obstacles. Ropes and chains, stacks of junk, anything to stop a dragon fully spreading its wings. Keep him uncomfortable.

Collapsing ceilings on corridors can act as traps or barriers, whether to keep a dragon in or out.

• How many tons of rubble can a dragon effectively push or dig through?


• How much of the mountain do you need to drop on its head to kill it?

After that it's all about small spaces that dwarves can attack from but a dragon can't attack into. Consider the length of the head for example:

• If you make a space in this shape, can the dragon insert its head in such a way as to attack this space effectively?


• If you make it this shape, does the dragon's jet of flame follow the corridor round and hit it in the side of its own head?

Remember dwarves are the great blacksmiths and miners of the world, there's nothing they don't know about both moving rock and handling fire. Creating traps such that if the dragon blows fire into the wrong place it ends up toasting its own ears will greatly limit its freedom of action.

Murder holes, but rather than murder holes in the ceiling as usual, put them in the floor so dwarves underneath are poking holes in the vulnerable underside of your dragon.

n.b. A dragon thousands of years old is likely to know that dragons don't get to live for thousands of years by flying into dwarven fortresses and taking them all on head on. I'd expect something much more subtle than a frontal assault from this old beast. Dragons are cunning and have the wisdom of age, people are foolish, beware infiltrators and traitors.

Blow it up.

Dragons are big, but plenty of animals are big and still get killed. The distinguishing feature of a dragon is flame. A dragon doesn't win by being big, it wins by shock and awe, and because flamethrowers are spectacularly effective at room-clearing.

The problem for a dragon underground is that its opponents are in control of the air supply. And whilst it has a flame-proof mouth, it is not explosion-proof.

Firedamp is a major problem in mines. Usually you want to let the ventilation systems dissipate it cleanly. But if you know you've got a dragon coming, smart miners might store it. When you hit a pocket, you might simply seal up that part of the mine temporarily, with a gas-tight cover. Repeat until you have a whole bunch of sealed-up areas filled with firedamp, with ropes attached to the gas-tight covers. For bonus points you could leave open an access door around that area, kind of hidden but not very well, so you know where the dragon will be entering. Once the dragon's in there, shut the outer doors and use the ropes to open up all the pockets of firedamp. Then just wait for the bang.

For more technologically-advanced dwarves, this can be taken to new levels. Their mines will most likely have forced-air ventilation. If they have some knowledge of chemistry, they could easily pump methane or other explosive gases through the ventilation system. Again, wait for the bang.

And even more amusingly, you could have it burn itself up. Whilst a dragon's mouth and guts may be flame-proof, they're only proof against burning in atmospheric levels of oxygen. Pumping high levels of oxygen down the ventilation system will ensure that anything which burns will do so in spectacular fashion. The Fire Triangle means you don't need to supply extra fuel if the dragon's already supplied its own; all you need is extra oxygen.

Greatest disadvantage for Glori is that he is alone. As such, multiple methods may be implemented against him.

• Any known straight tunnel may be closed from both ends trapping it effectively without chance for diversion.
  Given the doors hold against it, the chamber's ventilation the may be sealed to kill Glori by suffocation.


• Drapes of chain may be hung from open spaces to hinder flying, diverting its path as needed.


• Flying exposes the underside of Glori, giving dwarves chance to turn it to a pin cushion, given dwarves use bows or crossbows.

These methods may result in massive losses for the dwarves while attempting to manipulate the Drake, but the siege itself will ultimately fail.

Why defend? By all means, make sure he comes, tell everybody about your treasures if nothing else helps. Having an endless source of fire in your city is what every dwarf dreams of. Never have to light coals and pull the bellow again if you feel like smithing. Cold winter? Fancy a hot bath? No need to collect fire wood.

Now the only tricky part about it all is keeping him. Dragons are not well-known for being amused by providing free fire or by their general courtesy, for that matter.

Thus, the bait treasure needs to be placed where there is only a single point of access, and the passage needs to be able to be barred with fireproof (sliding rock wall 2-3 meters thick?) doors. Something like the seal-off mechanism in pyramids as seen famously portrayed in the 1955 movie "Land of Pharaohs", only maybe closing the passage a tidbit faster.

You will likely want a couple of small (too small for a dragon) vents going to the smithy (or city warm water facilities) near the front so you can harvest the fire. And, of course, an opening somewhere at the rear so you can pull the dragon's tail or poke him with a pointy stick when you need fire and he doesn't feel like exhaling.

The Kingdom of the Dwarves has been rich and prosperous since...

We all know where this trope goes.

The dwarven army will never defeat the dragon by itself. They may not be ninjas, but they are still affected by the inverse ninja law:

The inverse ninja law is a media trope regarding not only ninjas, but any character type that is shown to attack in massed numbers, such as soldiers, robots, daleks, or vampires (but not zombies). It states that the threat level of any number of ninjas or other whatsits is inversely proportionate to their numbers. Therefore, if you're attacked by a lone ninja, you're in trouble, but if you meet an army of ninjas, they're going down.

What they really need is an adventurer who will be more glad to defeat Trog Dor Glori just for glory, gold, fame, or a combination of these. In the very least the gold and magical items ar disposal of the dwarves should be plenty. It's just a matter of establishing compensatiom for the hero services.

If not a single hero, then a party of player characters adventurers can bring in a mix of talents that a bunch of nameless NPC's the dwarven forces lack, thus presenting a greater threat to the dragon.

These dwarves have had warfare before as evidenced by the presence of a military. Unlike a castle where you fill open space with stone, dwarves fill stone with open space to build their fortifications. This means you control every entrance the enemy has and what the entrance looks like, and the dwarves will have made use of this. I am assuming the dwarves have very little time to prepare as the dragon will travel faster than the news of its possible destinations, so only ad-hoc defenses or defenses that would be expected against "normal" armies.

Entrances might be wide avenues for the trade and traffic to go through, but the surrounding area can be hewn any way you want.

• The walls lined with arrow slits for (X)bowmen to Use? Check




• "small" artillery like Ballista's to shoot any enemy siege engines like a rolling wood&metal arrow shelter? Check. Hey this would fit well against a dragon




• Murder holes in the ceiling where things can be dropped from, like chlorine gas (heavier than air) to suffocate, boiling water, tar, burning oil or molten lava/metal if not normal darts, arrows, small rocks or boulders big enough to crush any siege engine (or dragon) driving through it? Check.




• a tunnel below the entrance where fire can be lit and holes in the floors where the smoke can rise through to reduce vision and suffocate the enemy? Check. The tunnel could also be used for a bunch of spearmen to stand below and stab upwards of luckless enemies walking over.




• A slightly slanted floor with giant metal balls at one end to crush anyone trying to move up the entrance? Check. Alternatively the entrance can be sloped downwards and the balls are released from the ceiling of the entrance, possibly rolling across the walls from the sides so the balls can more easily be collected and repositioned.




• Large contraptions designed to kill. If these dwarves are the more technical kind that use massive contraptions to haul ore up, use elevators or similar tricks you can power killingtools with them as well. A giant blade that comes from a slit in the wall and cuts a squad through could be a boon against a dragon




• Designed collapses to bury enemies.




• The standard tricks of the trade from chokepoints to ambushes and whatnot.

Here are some tactics:

• Trap the dragon. Have chambers and tunnels that can be sealed so that the dragon cannot get out. Put murder-holes in the bottoms of the trap-tunnels, to attack the dragon's underbelly with spears.




• The dwarves keep, or hire, large mongeese or ferrets to hunt down giant snakes in their tunnels.




• The dwarves snuff out the dragon's flame. Perhaps they use giant bellows to pump out all the air from a portion of the city. Or perhaps they use backfires to use up the oxygen.




• Flood the chamber that the dragon is in.

Use a Guillotine

Move all the treasure to one dead-end cavern. Leave all doors open in the rest of the system.

Block the treasure cave entrance with an important-looking locked door, made of wood.

Behind the door is a huge raised guillotine blade that is triggered by an automatic mechanism or by a few volunteers.

Evacuate the rest of the cave system to a nearby forest where the dwarves can camp until Glori is dead.

Mr Miyagi's best defense: No be there.

Fake city.

Inspiration: the antitheft trick where you have a wallet with $42, a photo of Zsa Zsa Gabor and some useless cards. When the robber demands your wallet or the pickpocket snakes it, that is the wallet they get. Your real wallet is deeper, in a fake interior front pocket.

Glori has heard rumors, but she has never been to the City Under the Mountain. The apparent entrance to the city leads to a City Under the Mountain, OK - a halfassed city but actually not even half - a great room with some 4th rate treasure, a couple of buildings and tunnels, and that is it. Glori will not be surprised that the dwarves all run away when she shows up. She might be disappointed that the city is not all it was cracked up to be, but she understands how legends work. She is content - it is nice enough, and she took it fair and square. She will curl up on the mound of gilded lead and brass (which is arranged in a surprisingly comfortable way!) and go to sleep.

Those with a discerning eye who explore the halfassed fake city will note that the sculptures decorating the interior lack the decorum one expects in actual civic artwork. Dwarven civic sculpture usually depicts individuals fully dressed and facing forward. Glori does not have a discerning eye for dwarven sculpture.

One of the critical factors in defending against Glori and his ilk is going to be the presence and availability of weaponry that can hinder or injure him. While enough sturdy armoured Dwarves may be able able to swarm and crush him under sheer weight of numbers, doing so against a fire breathing foe in cramped tunnel conditions is going to get real ugly, real fast.

Small portable siege engines, such as the Ballista or Scorpio may be able to pack enough punch to penetrate the thick draken scales. These could be set up at choke points in the tunnels, as well as key locations in the city itself to provide cover against an airborne fore.

Fortifications could be designed to be more resistant against a fire-breathing enemy, with armoured hatches over arrow slits and doorways

Traps could be set at likely locations in the old tunnels. A skilled stonemason could even set a trap such as an ordinary dwarf could walk through unharmed, but any creature the size and weight of a drake would cause a cave-in, burying the beast under tonnes of rubble and stone.

I find it wrong that this question assumes that Glori would implicitly have higher hand. To me, there are too many variables to determine who would win and what tactics would be superior.

The most important part is preparation. Both sides's level of praparedness can range from voefuly unpreprated to crazy prepared.

If Glori is not stupid, he would spend lot of time and resources to prepare for his assault. He could gather information on layout and traps of the fortress by either stealing or buying blueprints or capturing or bribing dwarves. He would hire adventurers or mercenaries to help him in the assault. He would have equipment made to protect his weak spots. He would send spies to sabotage traps or open gates at the right time. If he was really rich, he could bribe nearby (Elven) kingdom to declare war on the Dwarves and lure their army out of the fortress. If he was patient, the whole thing could be done over many years, as he has plenty of time.

If Glori was trully intelligent and patient, he wouldn't attack the fortress at all. He would go there not as and enemy, but as a friend. The dwarves would be suspicious of him at first. But over many years, he would make effort to be seen as friend and good thing for the fortress. He would get acquinted by the nobility of the dwarves. He would slowly influence the events of the fotress as to gain more influence and make dwarven nobility be seen as incompetent and corrupt. He would take control of the military and treasury. And in the end, it will be the dwarf peasants themselves who would rather have him as a King and not some corrupt, incompetent Dwarf.

tl;dr; Glori ran Palpatine on the dwarves.

Rock is fireproof, if you get the right type. The dwarves don't know every way into their mining complex but they should know all the access ways into their active areas, where they live and process ore and other materials, and they certainly will know all the routes to their treasure vault(s). They should start by telling the drake exactly how to get to their treasure, then rig a "choke route" tunnel, one that the drake has to take no matter how they enter the complex, with a sheath of Mica, which is extremely fire resistant, and thick slabs of the same at each end that can be dropped in to block the tunnel off. Then they can trap Glori as she attempts to get to the vault and let her starve. They could also coat the mica in something that's toxic when burned to speed up the process if Glori tries to burn her way out.

Crush it

The dwarves are masters of stonework and construction, it is likely they will have ways to strategically cut off enemy approach by collapsing tunnels.
Even if a cave in doesn't kill Glori, it could trap her giving the dwarves a change to attack the prone drake, or prepare larger weapons and defences.

The entrance should be small and narrow, and in the entrance there should be a hoistable wall and after that wall, yet another hoistable wall that cannot be hoisted before mentioned the first wall has been unhoisted.
I hope that makes sense.

EDIT:
I was imagining an entrance to the mountain carved to fit the size of 3-4 dwarves standing shoulder to shoulder and maybe the height of two dwarves on top of each other. Now this solution would only be to keep the dragon out of the mountain and would require two iron/metal walls, standing about 3-4 meters from one another. The idea is simply put the same as the doors to secure research facilities (or so I imagine) where the second door will not be able to open, before the first door is properly shut. This would provide more entrance security for the dwarves, but would also eliminate the threat from the dragon, since they are, as OP explained, very long and would therefore not be able to fit in the gap between the walls.

With the enginuity of the dwarves this solution should be feasible.