As is said already: Page 1 is bigger than the adventure. The map itself needs a few rooms more, some hallways, perhaps a tower with something in it.
I think this could’ve worked with a twist: The players are not the thieves, but any number of hook-employees run against the players who are the disguised guards, trying not to blow their cover. A big table of events happening on every corner of the big map during the night (fixed and random) till the duke appears in the morning to celebrate the hymn.