I don't think storing it this way would fly in europe, and our walls are normally more robust than the average US wall.
The walls themselves are held in place by downwards pressure of the weight of the roof and the part of the wall above, and i'd say that fine grained salt behaves a lot like a liquid, therefore one must assume that the wall must be designed to withstand outward pressure of a liquid with the specific weight of salt. Whoever approved that to that filling height was pretty much lost to brainrot.
Since street crime rates in Vienna are very very low - and i don't look like an easy target - i am not anxious about that at least lol