On the overland map, a unit with Mountaineer pays no more than 1 Movement Point when moving into a Hill, Mountain or Volcano tile. When the unit is part of a stack, the entire stack pays no more than 1 Movement Point when moving into a such tiles. This allows stacks led by a Mountaineer unit to quickly cross mountainous terrain.
Unfortunately, Mountaineers have difficulty with plain and level terrain. The unit, as well as its entire stack, must pay 3 Movement Points to enter Grassland tiles (except those containing a Sorcery Node), making movement through such tiles undesirably-slow.
There is no known method of adding Mountaineer to a unit that does not possess it by default, whether by spells or through Magical Items. Of course, the Pathfinding ability, which can be added to any unit, is superior to Mountaineer in every way.
Intelligent creatures will normally adapt to their surroundings, especially over many generations. Some cultures specialize in survival in areas which are normally considered too harsh for us humans. This specialization allows them to live off inhospitable lands, and to navigate across regions that would seem uncrossable to others.
Mountaineers include primarily Dwarves, Giants and some Orcs - all of whom have a good familiarity with mountainous areas, and feel quite comfortable passing through such areas. Unfortunately, much like humans would have trouble navigating these regions, Mountaineers find themselves at a loss whenever confronted with flat, open areas.
When a Walking unit with Mountaineer is moving across the overland map, it can enter any Hill, Mountain or Volcano tile at a cost of only 1 Movement Point. Normally, Walking units would pay 3, 4 and 4 Movement Points to enter those tiles (respectively).
On the other hand, when the Mountaineer unit attempts to enter a Grassland tile, it must pay 3 Movement Points (where most other units pay only 1). This reflects its disorientation when traveling through open terrain with few landmarks. The only exception is the magical terrain of Sorcery Nodes (reported as also being Grassland by the Surveyor (F1)), which Mountaineers can enter for the same 1 as any other Walking unit.
Mountaineer does not change the Movement Allowance of the unit - only the number of Movement Points it must pay to enter a given tile based on its terrain. Mountaineer also only applies on the overland map. Units possessing this ability do not behave any differently from other units when fighting in Hill, Mountain or Volcano tiles.
Note that any Flying units with Mountaineer do not feel the aforementioned effect. Their movement rate is 1 Movement Point per tile, regardless of terrain. Mountaineer only assists ground-moving units.
Group Mountaineering Edit
A Mountaineer unit traveling as part of an army will bestow the effects of this ability on all units stacked with it. As a result, the entire stack will pay only 1 Movement Point to enter a Hill, Mountain or Volcano tile, while paying 3 Movement Points to enter a Grassland tile (again, with the exception of Sorcery Nodes).
If the stack contains only Flying units, the Mountaineer ability has no effect: the stack will pay only 1 Movement Point to enter any tile, as is normal for Flying units. However, if the stack contains at least one Walking unit, the movement costs described above are in effect. It does not matter which unit in the stack possesses Mountaineer.
It should also be noted that when a Mountaineer unit is traveling as part of an army and there is another unit with the Forester ability in that same stack, the entire stack will travel using the Pathfinding ability instead (see the linked article for more information).
Units with Default Mountaineer Edit
Note that all nine units are Walking units by default (thus potentially enjoying the benefits of the Mountaineer ability, as explained above). The two Heroes arguably benefit much more from Mountaineer, as Heroes' Movement Allowance can easily be increased with the use of Magical Items - allowing them to cross vast distances rather quickly if ample mountainous terrain is present.
Acquiring Mountaineer Edit
There is no known method in the game to add Mountaineer to a unit that does not possess it by default.