If we grasp around what we or what is considered the shadow plane for a moment. Would Sigil somehow exist within it somewhere. And further more, if it did, would this, bear any light as to why the lady's shadow has the properties of a ticked off Githyankee and a bag of really tiny darts. But more to the point. How does anything cast a shadow in sigil from a place that has no obvious light source.
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It shouldn't unless a building is somehow blocking light.