I remember reading a book about four children who go to their uncle's house and find a bunch of TVs unplugged in the attic. They are able to go through the TVs into an alternate fantasy world, where they appear in a prison and meet an old man. Some scenes and facts I remember:
Pool/well with spiral stairs leading down to a door at the bottom of the pool.
Castle island in a lake
Father time personified along with a sleeping army
Main character having to navigate tunnels with red and blue lights acting as guides
One of the 4 children was a little girl
Pretty sure the title had the word tower in it!
P.S. It's not Narnia for sure
Any help would be appreciated! I've been looking for years!