It seems the Pirates can't build ANY offensive or defensive units. Is this intentional?
Pirates are better than other species in building naval units. But they also have ground units.
Pirates can build at the beginning of the game,
Light Spearman (Pirate Traditions) .
Later ground attack and defense units:
Light Swordsman (Weapons) (even iron needed to build). United pikeman (Armor) (hardwood required).
Medieval Swordsman (Martial Traditions) (iron required)
Pirate(Pirates) (iron needed)
Pirate Man(Armament)(iron and saltpeter required)
Mariner(Warships)(saltpeter required)
and
War Dog(Bestiary)(wolves required)
With the monsters, expansion is pretty much not possible.
If you are thinking of building monsters, part of them can be built by all species.
Eg if you have a
centaur hut you can build centaurs (bestiary)
Faun can be built if we have fresh water
Gnoll Infantry can be built if you have a gnoll hut... and so on.
The first generation (weaker) dragons are created by certain Wonders(first generation dragons: Earth Dragon, Water Dragon, Air Dragon and Fire Dragon).
Stronger dragons can be built in the 4th era(
Amber Dragon, Obsidian Dragon, .... Diamond Dragon(raw material dependent dragon cave + gems)