I misinterpreted what the OP was talking about because in a post a few months ago, someone asked about the "White window" in front of the castle and actually was asking about the medalion where the pictures of the castle are. I have seen pictures of the window with a light on at night now and don't know why it's lit. But since the castle was built in 1969 and Walt died in 1966, the light may be a tribute to him, but isn't to light his former apartment.
From Wilkipedia.com :
The Disney Apartment
Inside the upper levels of Cinderella Castle, there is a penthouse that was intended to be an apartment for Walt Disney and his family when they were in Florida. After Walt Disney died in 1966, the apartment was left unfinished. Instead, the penthouse served as the operations area for Vista United Telecommunications (switchboard operators). It now serves as dressing rooms for the entertainers performing in the shows at the Castle Forecourt Stage.
and from
the DIS site:
One of the upper floors includes an unfinished apartment originally constructed for use by the Disney family. The room now serves as a change room for Tinker Bell as she prepares for her nightly flight.