From a bug report: zapping a wand of striking or locking or spell of
force bolt or wizard lock up or down when standing at an open drawbridge's
portcullis didn't affect the bridge if the portcullis was positioned north
of the open span. One of the two drawbridges on the Valkyrie quest goal
level has that orientation. is_drawbridge_wall()'s name is somewhat
misleading; it isn't boolean and returns -1 rather than 0 for "no".
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