Men of the West
The Men of the West are the combined forces of Gondor and Rohan. There are three separate structures that must be built and used to develop the full breadth of the Men of the West, including the Barracks, the Archery and the Stable. The Barracks trains melee-based foot-soldiers, the archery produces archers and rangers, and the Stable trains the vaunted Rohirrim calvary.
Elves
The Elves consist of the various troupes and Houses of elven creatures, including Hobbits. However, their armies stem from two structures, the elven Barracks and the Green Pasture. The Barracks develops all manner of soldiers, including swordsmen and pikemen, but focuses especially on differing archers. The Green Pasture is the source of elven calvary. It is also possible to turn an outpost into an "Entmoot" upon discovering it, which produces Ents, or walking trees.
Dwarves
The Dwarves are drawn into the plot of the game when the elves come to the realization that they are not strong enough alone to face the forces of evil. Dwarven strategy relies on three buildings: the Hall of Warriors, the Archery Range and the Forge Works. The Hall of Warriors deals with melee, the Archery Range with axe-throwers and human allies known as Men of Dale, and the Forge Works with siege equipment, among the chief dwarven advantages.
Goblins
The Goblins are paradoxically useful in that their bread and butter, their soldiers, are relatively weak and numerous, yet as a race players can summon the dreaded Balrog or fearsome dragons, which lay waste to a base single-handedly. Building a horde of bloodthirsty goblins requires the Goblin Pit, the Spider Cave and the Fissure. The Goblin Pit develops both melee soldiers and archers, the Spider Cave trains mounted units, and the Fissure makes siege equipment.
Mordor
The forces of Sauron are as various and ragtag, resulting in an impressive range of fighters. The Orc Pit provides warriors and archers to harass enemies early and often; the Haradrim Palace develops a finer class of pikemen and archers, as well as contributing huge elephant beasts; and the Troll Cage outfits Trolls to lay waste to enemy foot soldiers. Sauron also employs the Great Siege Works for war machines.
Isengard
The Uruk Pit and Warg Pit are the industrial forges of the Isengard armies. The Uruk Pit grows a range of melee, ranged and special units, while the Warg Pit is responsible for the faster and stronger Warg Riders. Finally, the forces of Saruman use the Siege Works to build war machines, though they are not very effective compared to those of other races.