You know, I got that inspiration that if dwarves are very class-concious and always follow strict professions and structures, I kinda should build the fortress like that. the first underground level will be built in earth or sand; this is the wood level. All furniture there will be made out of wood and only wood and other organic materials will be processed there. This is the center of food production and the early sleeping rooms. Even the chairs and tables of the dining room will be made out of wood and there are no coffins; only a graveyard. But it will also be the production center for clothes and leather; this will too be where refuse gets worked into totems and delicious booze will get brewed. It may sound like the least valuable part, but as every wood-Dwarf knows you can't survive without the wood level.
Digging deeper we will reach the stone level. There of course will be everything made out of stone. The Masons, the stone crafters, coal and gem workshops and proper stone dining rooms and throne rooms for minor dignitaries. Unfortunately there is still need for bedrooms for the stone-level dwarves, so their rooms will include stairways up into the wood level, but with no connection to the wood part of the fortress to provide them with a proper sleeping place. All this may not sound worth much, but the stone level provides very valuable support for the levels above and below. The sone crafts are a steady easy source of income for trading for food and weapons and jewelries improve the worth of every level. Not to mention the engravers recording the history in the very fortress. Every stone-Dwarf knows that this level distinguishes the fortress from a simple Elf village.
Even deeper we encounter the iron level, the only acceptable building resource of course being iron. Here we have the smelters and workshops to produce iron, steel, copper, bronze, silver, lead and all the other non-precious metals we need to properly outfit the fortress guard and to provide high-value dining and throne rooms for dignitaries. Here too will the stone crafts be studded with metal to improve trading value even further. This level may look like the most ugly, with strip mining sprawling downwards below the few pretty metal-furniture rooms, but the metal-Dwarves know that they alone stand between Dwarven prosperity and utter destruction at the hand of Goblins, Elves and worse evils.
Somewhere will be too a gold level. Just a few workshops and rooms shining with unmeasurable wealth from gold, platinium and aluminium that the iron-Dwarves find in their digging. The gold-Dwarves don't care what others think. They know that they alone can provide enough wealth to satisfy even the most valued dignitaries and if they bother to send a few trinkets to the trade post, they will be able to buy whole caravans.
Finally there are rumors about the famed adamantium level. Only one or two workshops working on the mystical ore and always the adamantium guard who keeps watch to protect from ancient evils. Should they fall, everything is lost.