There was a key reference to Roland and the Dark Tower in “Insomnia”.
The boy who was supposed to live was drawing a picture of a king and a tower. The mother asked who it was and the kid said “It’s king Roland”.
I think I remember that.
The Tower has been referred to in other King books, as well. It figured in Black House, the sequel to The Talisman.
And there’s something about a field of flowers...maybe roses?