Fortresses are, depending on who builds them, a fortification or settlement.
Dwarves tend to prefer to live underground or in mountains and hillsides underground. As thier capitals tend to become massive undergorund complexes, they have come to be called fortresses. They may or may not come with extensive surface elements (such as those seen in Spearbreakers ,) and are generally well defended and difficult to take due to a combination of traps and soldiers.
For most, a fortress is simply another name for a dwarven city, though thier major cities have the special name of "Mountainhall."
Human and Barbarian Fortresses
Humans and Barbarians build large stone fortifications near major cities and towns that serve as a combination military base and residence for thier nobility. human capitals are nearly always built around a fortress and barbarians tend to build thier capitals entirely undergound save for "Surface Sprawl" in imitation of the local dwarves.
Goblinoid and "Evil" Insectoid FortressesEdit
Goblinoids (which including hobgoblins and orcs ) as well as Violet Xelics tend to build thier major cities out of carved obsidian, though the latter two will eschew this practice if none is readily available. Goblins are known to pay hefty sums to other goblin nations, humans, and many others to bring obsidian in for construction. Goblin and Violet Xelic fortresses are known as Dark Frotresses.
Elvish, "Good" Insectoid, and Equus FortressesEdit
While it's not common for Elves and Silver Xelics to do so, they recognize the need to defend and watch over important locales in thier holdings and will build or grow entire fortresses from wood. While they are not as permanent or easy to defend, they are vastly quicker to assemble, using either Nature Magic or bringing wood in from elsewhere.
The Equus often do the same, preferring quick-to-build wooden forts with either a spike-filled dry moat or water-filled wet moat for protection. Unlike Elvish and Silver Xelic forts, Equus forts tend to also be settlements in thier own right and built from harvested lumber in the local area.