Dominating the western border of Skyrim, the Reach is made up almost entirely of steep, craggy mountains. Little grows in this forbidding realm, but the capital city of Markarth is a nigh-impregnable stone fortress that would make an excellent defensive position for either side in the war.
Be aware that this dangerous region of Skyrim is home to the Forsworn, the rebellious natives of the Reach. They know the terrain, can strike without warning, and count the Empire as an enemy. If they attack, you must neither give nor expect any mercy.