People like build base back to the mountain, it's easy to defend. But IRL, this has the harm of debris flow and avalanche.
So suggest to add debris flow and avalanche.
Then there is a chance to happen these disasters after heavy rain or snowstorm, and can destroy everything in the range of several cells at the foot of the mountain
Mortar hits on the mountain could cause small chunks of rock to cave in, would still give more protection from mortars than being outside but not entirely immortal.