Dwarven defence is unquestionably superlative to any other race in the game, but a few other points should bear mentioning.
"When night came on, bundles of twigs were fastened to the horns of some 2000 oxen and set on fire, the terrified animals being then quickly driven along the mountain side towards the passes which were beset by the enemy."
- Sun Tzu, The Art of War
First, on the subject of traps. Sending your soldiers into the tunnels to be slaughtered is poor strategy, and with the need for traps to be reloaded or reset in the event of a jam they become trivial to bypass. Simply taking a herd of your livestock and tying lit torches to their tails will allow you to drive them into the tunnels, triggering the traps with minimal casualties.
This is a good way to deal with forts that rely overly on the (admittedly overpowered) cage trap. But an invading army that sends most of their cattle into a fort with even a basic chasm will find that they've merely increased their opponent's meat stocks at the cost of their own. Generally, your first line of serious defense should be something that doesn't saturate easily, or even improves with volume; the Durin's Bridge plan has always seemed a good idea to me. The more invaders that try and cram across, the more of them are likely to fall into the depths.
Of course, with the new climbing rules, it's more important to smooth your chasm walls and not skimp on the height. Giant cave spiders are even more terrifying than they have been. The only *serious* threat to a well-organized mountain hall are fireproof flying building destroyers, either in quantity or with trap-avoid.
"Logistics comprises the means and arrangements which work out the plans of strategy and tactics. Strategy decides where to act; logistics brings the troops to this point."
-Jomini: Precis de l' Art de la Guerre. (1838)
One of the biggest advantages in medieval warfare came from supply line logistics. Without a steady supply line, the army would run out of food and materiel well before encountering the enemy. The game abstracts the concepts to a great degree with invaders lacking requirements for food or drink, but in terms of dwarves in their mountainhomes the biggest advantage they have is the ability to feed the fortress from within. However the supply of fresh recruits to replace lost military requires breaking the siege, as without doing so no migrants will emigrate.
I think you underestimate the power of DF dwarves being able to feed themselves in a closed ecosystem. Both historically and in middle-earth, sieges could last years against durable fortresses that simply couldn't be taken; but even Khazad-dum didn't produce enough food to feed itself. No bastion of conceivable scale could both be impregnable against assault, and large enough to be self-sufficient in food and water. But besieging a DF mountain hall is more like besieging the drow in the Underdark; they can "live off the land" better than the invaders can.
All mountain halls, by contrast, can have unlimited food, drink, and magma. Most have unlimited water, which also means unlimited stone, and unlimited glass is not uncommon. Unlimited wood is doable, although production rates are possibly a concern; similarly, there's no limit on dwarves themselves, but production is slow.
The degree to which an established mountain hall is self-sufficient is unprecedented in reality, and even in most fiction. The vast majority of foes will expend far more resources on maintaining a siege than the dwarves do in withstanding it. And remember, the dwarves are at home living their everyday life, and you are in the field, far from fields and family. Mortal besiegers need to worry about how the rest of their kingdoms will fare if they plan to spend decades surrounding a mountain that doesn't seem to even acknowledge they're out there.
Of course, eventually foes capable of even slow hard-rock mining will probably be in the game. Humans may rarely be up to it, but the immortal races can take their time. "How are the mines coming?" "Oh, we're getting at least a square per season, sometimes two; the new shaped-wooden picks are doing much better. I figure we'll have a gallery driven to separate their citadel from the magma in well under 500 years, and barring anything unexpected should have this won in three, maybe four thousand years tops. What are you doing for lunch after?" Of course, "something unexpected" may well be the dwarves finishing their "this mountain didn't start as a volcano, but it is now" megaproject, and flooding the world with lava. Eventually besiegers will learn that they need substantial dikes or a height advantage.
I expect that against high-tech foes (currently, only rival dwarven kingdoms) we'll eventually end up with something like the
Siege of Turn but in 3D, with elaborate mines and counter-mines eventually turning the entire mountain into an anthill. Systems to "regenerate" the mountain will become increasingly important.
Also it should be noted that dwarves, for all their skill with underground living, have little traditional naval warfare skill, and an invading army supplied by sea without any threat of retaliation will have strong logistical support.
That varies with setting; note that *no* races in DF currently have naval warfare skill
In some, dwarves are Viking-like, and the terror of the northern seas; we'll just have to wait for it. Then there's the whole "who needs boats if you have unlimited magma" approach. To a dwarf, it's possible that wasting time with boats would look like fooling around with zeppelins do to us; unstable, weather-dependent, low load ratings, and with an unfortunate tendency to burst into flames at a bad time. Why, for a small fraction of the effort needed to create and train one of these "navies", you could have obsidian bridges and tunnels humming with minecart traffic. (Or to put it another way, for dwarves it may be more logical to build the Chunnel than bother with all that liquid sloshing about up top.)