Jesus never really defined the kingdom in the Gospels, rather He presented it as "a riddle". Let's examine this aspect:
The Kingdom is something "so gigantic that the whole world can enter into it... each one of us has to enter into it (Jn.3:3,5, Matt.18:3).
But the kingdom is "so minute that Jesus enters into each one of us, it is within us (Luk.17:21, Matt.6:10).
So the obvious answer is the kingdom is not so much a place but a person, and that personality is "Jesus Christ. Jesus is the King and the Kingdom: Jesus not only came to show us the Way, but He Himself is "the Way", I am the way and the truth and the life (Jn.14:6)... and He Himself is "the Truth" and "the Life"... ... In the Kingdom of God, Jesus is the King... and He came to preach the Kingdom... but not only that, He Himself is "the Kingdom". What are your thoughts?