Board Thread:Lore Discussion/@comment-24627553-20140316230248/@comment-1600847-20160921175242

I believe he means the following passage from The Book of the Dragonborn:

"[[When the  Snow Tower  lies  sundered, kingless, bleeding]]"

Specifically, "Snow Tower lies sundered", which would suggest the Tower was deactivated when Ulfric killed Torygg and started the Civil War.