Herein lies the actual crux of the issue with communism.
Humans are NOT wired psychologically for it. Humans seem to have this intrinsic drive to compete, and gain "status". In a communist setting, (like the REALLY REAL thing Marx wrote about) there is no room for this. There is no such thing as "Status"-- everyone has equal rights and equal access to resources.
So, to PREVENT people from hoarding resources, and INVENTING something to have status over, ("I have the biggest X!!" / "I have the most Y!" / "My inventions are so much better than HERS, so I'm the ELITE one!" etc) which is often a completely arbitrary thing-- (It's basically what is underneath racism-- something as absurd as skin color used to define status.) a new caste must be created to enforce that equality--- And yes, THIS IS ABSURD. IT DEFEATS THE PURPOSE. (You create a government sanctioned/empowered demographic that really DOES have unequal power and access to resources, for the express action of trying to prevent such a group from forming! ASININE!)
It is this latter, unholy perversion that is what prevents communism as marx envisioned it from working. It is why communism is synonymous (in the public eye) with totalitarianism-- without the secret police, the people in the society will not uphold the foundational tenets, and the society collapses --With the secret police, it is no longer true communism. Humans just cant have nice things.
This does not mean that dwarves have the same "OMG, I just GOTTA be the PRETTIEST PINK PONY OF THEM ALL! The PRETTIEST Gawd Damnit!" neurosis that humans seem to have burned into them at birth. As such, dwarves may well be capable of having true communism. (However, as pointed out by others, the existence of nobles basically demonstrates that they indeed DO have this issue, and therefor, CANNOT have true communism.)
Kobolds, of all things, seem to have the most earmarks of being truly communist of the races presented in DF-- No leaders, no concept of ownership, no government, no laws, and yet very little if any in-fighting. They have the system already, and do not require regulation to sustain it-- Naturally Communist. The real deal.
Dwarves strike me as favoring fascism, with a strong dictatorial executive center, coupled with strong, state-run industrial power base.
Humans are more feudal society, with a monarchy, but with competing industrial power that works independently of the monarchy (and may even subvert it on occasion!)
And elves are a straight up theocracy, where religion and law are combined into one practicing body with absolute authority.
Due to this, To me, an ideal fortress (for dwarves) consists of a strong nationalist identity, (A civilization), that has strong affiliations with industrial power, (State-mandated and controlled guilds, who maximize government efficiency within their realms of expertise, and who directly influence policy decisions), a strong national military machine, very strong nationalist pride, and rigid divisions between tiers of society and power hierarchy. Such a fortress will by necessity have a strong fortress guard that secures and maintains these divisions, and the creation and maintenance of this guard will not be anti-thetical to the underlying ideology behind their form of government. Naturally, they will believe that their way of doing things is without question "the best there is."
[In so many words, dwarves appear to be fascist, not communist, as pointed out earlier.]
This is why I view the "The underbelly of fascism, when it all goes to hell" type scenario, with the state police holding people against their will inside the nation, and hyper-inflating the nationalist propoganda and nationalist militarism in the face of systemic urban unrest, complete with brutality by the fortress guard for people who would DARE question or countermand the central authority as the ultimate DF fortress challenge.
Such a challenge would:
1) Eagerly take *ALL* migrants, as long as they were from the *RIGHT* civilization. (All aliens are killed on sight for being pollutants.)
2) All migrants that arrive MUST STAY INSIDE THE WALLS, or be shot for being disloyal.
3) All disagreements with central authority on these matters is a sign of festering disloyalty to the nation, and is met with capital punishment.
4) Due to the fusion of industrial power with government power, workers who do not meet requirements for quality and time vs resources consumed, are punished-- where those that can exceed expectation are rewarded-- within the boundries afforded to them by their station.
5) Mandatory service in the national military machine is mandatory. As above, poor performers are penalized.
6) To demonstrate the CLEAR superiority of the nation, militaristic actions are routinely carried out to secure advantage for the CLEARLY BETTER people of their civilization, at the detriment of all other nations.
7) Some arbitrary metric for deriving status must be blown completely out of all sensible proportion, and be used exclusively to determine social status stratification.