I'm considering these options:
1. Introducing falling damage
2. Pregenerated Invisible walls
3. Considerable slow down of running on impassable areas
4. Dynamic system that will evaluate terrain steepness and will force you to stop before "dangerous" areas
5. Combination of these approaches

Each approach has its pros and cons so I will appreciate players feedback here.

Thanks
Peter

I would like combination of 4 and 1. Maybe something like a warning which we can choose to ignore at the risk of sustaining injury. Although invisible walls would be easiest to implement probably, I hate them in games:)

If you will allow to ignore that risk than you need system will always "guarantee" an damage to player. Which is not easy as often you can go very slowly through steep angles to avoid fall damage. Or you need to counter that with slipping which may not work at 100%

Option 3 would counter this. Well, you would cross an uncrossable area. But at least you wouldn't win the competition. But it's probably even less nice option like invisible walls.

Maybe there should also be map option for 2 vs 3?
It can be handy in future for maze-like maps, where relatively narrow impassible areas should not be passed through (i.e. symbol 411 ISOM 2017)