Crusader Kings II features different terrain types, as well as oceans and seas. Each type of terrain has its own effect on your army. Terrain affects your army's supply limit, movement speed, attrition and defence (for the defending troops).
You can view each county's terrain type by switching to the terrain map mode (lower right corner) and hovering your mouse over the map. A tooltip will tell you the terrain type.
Following are the terrain types and their modifiers.
| Terrain type | Modifier |
|---|---|
| inland ocean | movement_cost = 1.0 |
| arctic | movement_cost = 1.0 max_attrition = 0.1 supply_limit = 1 |
| farmlands |
movement_cost = 1.25 supply_limit = 5 |
| forest |
movement_cost = 2.25 supply_limit = 3 defence = 2 |
| hills | movement_cost = 1.66 defence = 4 max_attrition = 0.025 supply_limit = 3 |
| woods | movement_cost = 1.1 defence = 1 supply_limit = 3 |
| mountains |
movement_cost = 3.0 supply_limit = 2 |
| impassable mountains | movement_cost = 8.0 defence = 6 |
| steppe | movement_cost = 1.0 supply_limit = 4 |
| plains | movement_cost = 1.1 supply_limit = 5 |
| jungle | movement_cost = 3.0 defence = 2 max_attrition = 0.05 supply_limit = 2 |
| marsh |
movement_cost = 2.2 supply_limit = 2 |
| desert |
movement_cost = 1.25 supply_limit = 1 |
| coastal desert | movement_cost = 1.0 max_attrition = 0.05 supply_limit = 2 |
| ocean | movement_cost = 1.0 |