As the military and economic center of the largest Orc cities, the Fortress
can hold and process all the gold and lumber that the Peons can be
made to harvest. Protected by obsidian spires shaped from the earth
itself, the Fortress is all but impervious to the attacks of the feeble
Human forces. Constructed by Orcish leaders attaining arrant mastery
both on and off the field of battle, the Fortress serves as a proclamation
to the Alliance of the inevitable triumph of the Orcish Horde.WC2BnetMan 81
Once orc technology reaches its height, construction of the
Fortress may begin. Far more formidable than even the stronghold,
the fortress provides a siege-resistant command base. With
the addition of spiked barricades, a Fortress may daunt even the
most hostile foe.WC3Man 28
Known Fortresses
Blackrock Spire (speculated, but represented as such in-game, and obviously similarly volcanic)