The Rayonnant style of Gothic architecture was known for its:

dark opaque glass windows
majestic tall spires
radiating patterns of rose windows
low, dimly lit sanctuaries

Explanation:

It refined elements of Gothic style like tracery, the supporting stonework between window segments so that massive churches had a sense of lightness and delicacy. Rayonnant architecture gave prominence to massive areas of stained glass windows and rose windows, which are large circular windows.

your word would be isle.

isle is an island or peninsula especially a small one.

if you add the letter "a" you get aisle, which is a passage between 2 rows of something like chairs, shelves, or something else.