The series follows the journey of Roland Deschain, a gunslinger of Gilead, in pursuit of the fabled Dark Tower through Mid-World and on the trail of his sworn enemy, The Man in Black.
He walks in pursuit of The Dark Tower and his enemy, The Man in Black.
Yes, The Dark Tower is an incredibly meta fantasy series and even ropes author Stephen King into a pivotal plot point.
The Crimson King is an evil entity that hopes to bring down The Dark Tower.