The Westerlands start in control of Kings Landing. It also has very prosperous towns. It can be recognized on the default map by its red colour. Its capital is Lannisport.
Early in the campaign, the Westerlands finds itself beset on all fronts. It starts out at war with the Riverlands and the North, and soon after, the Stormlands, the Reach, and Dragonstone will all declare war on the Westerlands. If the player wants to help the Westerlands survive this pressure, an excellent way to do so is by triggering the Red Wedding. By doing the "Beyond the Wall" quest early on, many lords from the North will defect to the Westerlands and the rest of the North will declare a truce; they tend not to break this truce because they soon go to war with the Iron Isles and have their hands full. Northern troops and the additional lords definitely help keep the Westerlands from falling apart and are particularly fond of attacking the Riverlands, potentially removing the northern theater from the war entirely. After this, focus your efforts on combating the forces of the Reach, as they are the richest remaining faction and are only at war with one faction (you); subsequently, unless checked, they tend to carve territory out of the Westerlands.
Westerlands troops are known for two things: shield-bearing infantry and a decent archer line. Though the Westerlands has access to the Vanguard (two-handed infantry), this only comes at Tier 5, with most of the rest of the infantry being one-handed and shield-bearing. Unlike other factions, they can upgrade a Tier 1 Levy directly into the archer line, getting elite archers by Tier 4. Westerlands troops also tend to be slightly lower-quality and less expensive than their counterparts, making larger armies both more feasible and more necessary, especially against more elite armies like those of Dragonstone.
An early-game Westerlands army is a good enough "standard" army. It can hold a line well enough, though its archers should be given the greatest chance reasonable in order to weaken the enemy. However, their battle lines tend to fall apart under concerted pressure, so player intervention is likely needed. A late-game Westerlands army, is a juggernaut. The key ingredients? A)Heavy cavalry. B) Almost everyone is armored. Westerlands archers pepper the enemy (with the shield-bearing infantry absorbing return fire) until they are close enough for the heavy cavalry to smash into. The infantry then follows into the gap, cleaning up whatever is left. The entire time, arrowfire, spears, and swords stick in Westerlands shields or have much of their force absorbed by Westerlands armor.
Most early-game armies, especially those with heavy cavalry, tend to break mono-Westerlands battle lines unless they are greatly weakened by archer fire. Most armies will crumple when facing late-game mono-Westerlands charges and melee, unless they have good anti-armor.