Hi, I saw this Question in the Manhattan GMAT book.

Each of the following sentences contains one or more underlined modifiers. For each of these modifiers,
(1) identify the word or words, if any, that it modifies, and (2) indicate whether the modifier is correct.

If the modifier is incorrect, suggest a way to correct the error.

Question:

Upon setting foot in the Gothic cathedral, the spectacularly stained glass windows amazed the
camera-wielding tourists.

The Answer in the Book is
Upon entering the Gothic cathedral, the camera-wielding tourists were amazed
by the spectacular stained-glass windows.


My Answer is
Upon entering the Gothic cathedral, the camera-wielding tourists were amazed
by the spectacularly stained-glass windows.


I am unable to understand what is wrong with the "the spectacularly stained-glass windows"

According to my understanding :-

Stained is the adjective and spectacularly is an adverb modifying the adjective.
Glass is the adjective modifying the noun windows.

Can somebody please help me understand the error.

Thank You.