This is a Rohan deck enabled by Théoden, meant to be paired with a Gondor deck. The Gondor deck will make Theoden Gondor, giving him a slight WP/Attack boost, and possible also boost his defense and willpower other ways. Theoden's primary job is defense, but with Heir of Mardil (other deck) and Snowmane he will also contribute to questing and with Herugrim can contribute attack as well, if needed.
For the most part, enemies should be engaged by the Gondor deck, and Merry can use his threat reduction to keep the threat from getting too high. Dori and Theoden will defend across the table separately or together, depending on enemies. Most of the attachments that can will go on Theoden first, the exception being Gondorian Fire (for Boromir). Most of the allies in this deck are meant to quest.
Gamling is a key card for the opening hand, since he will enable getting back the self-discarding characters, or anyone dismissed with Ride to Ruin. But Snowmane or Unexpected Courage is good for the opening hand also. Westfold Horse-breeder can help get Snowmane and Hobbit Pony out.
In the sideboard Power of Orthanc is for conditions, Feint is for combat heavy scenarios were Theoden/Dori can't handle the defense load, and The Galadhrim's Greeting is for high-threat scenarios. Northern Tracker is for location-heavy quests. Hasty Stroke isn't present because of Balin.