Which risk increased when siege engineers of antiquity attempted to undermine castle walls past the 'angle of repose'?

💡 Explanation

When undermining castle walls beyond the angle of repose, the soil's shear strength becomes insufficient to support the overburden weight, leading to tunnel collapse due to gravity exceeding soil resistance. Therefore tunnel collapse risk increases, rather than explosions, flooding, or suffocation, which require different environmental conditions and extraction processes.

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