When moving up or down steep terrain, the fatigue/stamina cost increases when the slope of the terrain gets above a certain limit.

OBS:
The test for stamina cost due to terrain steepness is made on the direction of facing.
If the player moves sideways or backwards up steep terrain, they do not lose stamina quickly. If they move backwards down steep terrain, they do lose stamina quickly.
This can lead to implausible behaviour, such as running up hills backwards to avoid getting tired.

EXP:
The test for stamina cost due to terrain steepness should be made on the direction of movement (work done). Direction of facing is irrelevant.
If the player moves sideways or backwards up steep terrain, they should lose stamina quickly. If they move backwards down steep terrain, they should not lose stamina quickly.