Rooms or compartments with strong doors used to store valuable things like money or jewels, or arched roof structures in architecture.
From Latin 'volvere' meaning 'to roll,' because arched vaults are shaped like rolled or curved structures. The security meaning came from using these arched stone rooms in castles and banks.
A vault's curved arch design is pure physics genius—the arch shape means weight pushes outward and down, making it extremely strong. Medieval builders figured this out long before we understood the math, which is why old vaults are still standing!
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