The Stormlands are situated in a position quite central to the game, and border many other factions. When the player starts the game, they start in the Stormlands. On the default map, you can recognize the Stormlands by it's yellow colour.
Stormland troops focus on close-combat infantry. Though they do have a limited archer tree (tiers 2 and 3), their infantry tree is much more extensive (tiers 2-5) and leads to heavy cavalry at tiers 6, 7 and 8. Consequently, Stormland armies tend to close with the enemy quickly in order to (literally) hammer them into submission. Infantry comes in both one-handed and two-handed varieties. Unlike The Vale and Dragonstone, the two-handed infantry often carry warhammers and greatswords which are shorter than halberds. However, they tend to be better-armored, making arrowfire on an open battlefield less worrisome than it might be for a halberdier. On an open battlefield, a common Stormlands strategy is to advance with men-at-arms or other shield-bearing infantry first in order to absorb arrowfire; once the enemy are close (assuming they are not lance-bearing charging cavalry), Vanguards and Hammermen charge through the men-at-arms as shock infantry, with the men-at-arms cleaning up anything they leave behind.
One last interesting note: the Stormlands is the area where the player first spawns, making them a likely first component to your warband. To that end, Stormland troops make an excellent front battle line which can be augmented by the other factions, be they the spearmen of the Reach or the North, the archers of Braavos or Dorne, or the shock infantry of the Iron Isles. The one thing they do not counter well is heavy cavalry; they are short-ranged infantry without spears, pikes, or halberds, though if the cavalry become stuck in a mass of infantry, the two-handed Stormlands infantry will tear both mount and rider apart.
The Stormlands traditionally was the homeland of the Stormkings. After the Targaryen conquest, the Baratheon dynasty became the Lords Paramount of the region, seated in the famous castle Storm's End. During the reign of Mad King Aerys, Lord Steffon Baratheon and his wife died in a tragic accident. Their heir was the eldest of their three sons, the teenager Robert Baratheon.
Later, when Robert rose against the Mad King, the castle Storm's End was held by the middle Baratheon son, King Stannis Baratheon. When Robert's Rebellion succeeded, House Baratheon became the royal house. When Robert died, both his younger brothers rose against his heirs, whom they believed to be illegitimate. However, instead of rising together, they both pressed their seperate claims. King Stannis Baratheon rose, together with local lords and religious converts, from his seat in Dragonstone. Most of the stormlords however, chose to follow the younger Baratheon brother, Renly, who, though lacking the judical claim, pressed for kingship on grounds of his far larger popularity and his alliance with the Tyrells, who controll the Reach.