Spidersilk Has No Timer
Spidersilk in Shadowdark carries a duration of Focus, not a fixed number of rounds. As long as the spell is active, the caster holds concentration, and casting anything else drops it. That single detail turns Spidersilk from a free web into an ongoing expenditure. You are paying with your attention for as long as the silk stands.
The common misplay is casting it because it is on the sheet and the moment feels vaguely appropriate. Sticky terrain appears, nothing is coordinated, and the caster spends subsequent rounds babysitting a web while the fight moves elsewhere. Spidersilk earns its Focus cost when it dictates enemy movement, strands a threat out of reach, or closes a fight faster than it would otherwise close. Cast without a target and an intended outcome, it is a spell slot spent holding a doorway nobody wanted to use.
GMs can enforce the discipline the mechanic already implies. Focus is a resource, and a resource deserves a stated purpose before it leaves the caster’s hands.
At the table: Before the roll, ask the player which creature the silk is meant to stop and what they gain from tying up their Focus to do it.
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