von Marwitz
Forum Guru
Few people being able to shoot out, would not, however prevent many people from being inside the building.It's a castle with thick stone walls, and probably few windows. There would be no way to coordinate an entire platoon shooting from inside to the outside in such a building so the easy fix is to limit stacking.
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That said, if the intention of the SSR was to represent a scarcity of firing slits, then it should not limit the stacking limit but rather only allow area fire from the building. But even then - put a couple of MGs behind those few firing slits and you won't need a number of squads for the job.
von Marwitz