I like the general idea, and it would make sense, especially since it was originally a DotA feature. However, because the geometry seems to suggest an upward slope towards the riverbank in the spots you highlight, it would make more sense to only allow vision when immediately above those locations - i.e. you shouldn't be able to see over the ledge unless you're at the ledge. Same goes for the walls - because they're not just cliffs, they're actual walls on top of elevated terrain. But I do agree that if you're at a vantage point, you should have extended vision for the elevation.