The Kingdom of God

When Jesus was on trial before the Roman governor for insurrection, he told Pilate, "My kingdom is not of this world." What did Jesus mean? Is his kingdom just a "spiritual" kingdom? Is the Kingdom of Heaven just... Heaven? Or is the Kingdom present in the here and now? Is it a genuine power in our present, an alternative to all the powers we try to use to change our world for the better? What if the Kingdom of God is the most real kingdom of them all?