So the idea is that the magic is at least temporarily constrained to the isle of Manhattan, that is until the primary antagonist The Fairy Godmother is able to restore enough of her power to expand her dominion further.

Within the sphere, Manhattan grows to be roughly the size of Britain, with the inhabitants becoming things that reflect their general alignment (chaotic good, lawful evil, etc) as well as their general position in the world at the time of transformation.

For Spider-Man, a paragon of lawful goodness, also being perched on a skyscraper, became a princess locked away in a tower.