Board Thread:Questions and Answers/@comment-30535075-20180216171039/@comment-5398287-20180329192309

To illustrate what Sal said:

Chapter 523, page 10: Kenpachi enters Muken through a door

Chapter 618, page 4: Shunsui and Aizen specifically reference the "gate" to Muken, which Shunsui has the key to embedded in his heart

Maybe present some photographic evidence yourself before trash-talking people who are actually going by what was presented in the manga, AppleLord.